I'd probably be one stop less optimistic (a standard diffraction calculator is close to two stops less optimistic (f6.3 for 50 MP 24x36), but I'd split the difference - looking at large prints, I use my 46 MP camera very happily at f8, but NOT f11). 46 MP 24x36" is a reasonable starting point for two reasons. First, I have one and have examined the prints from it closely against 24 MP APS-C (which I also have in my bag) and 16 and 20 MP Micro 43 (have owned, and a good friend whom I print for shoots Micro 43 extensively). Second, a lot of LuLa folks have 40+ MP 24x36mm cameras - there are a lot of Sony A7r II and III users here, quite a few of us have D850s and an increasing number of Z7s, and the S1r is beginning to show up.
I agree strongly with your basic point, that resolution can't keep going up without the cameras becoming less and less practical to use. I'd add one additional factor to your calculations - lens performance. It's a lot easier to make a lens that resolves 50 MP at f8 (a good lens) than 200 MP at f4 (an Otus on a good day)!
With good sharpening, I'm VERY happy with prints from 46 MP 24x36mm at 24x36" (it was a revelation how much better that combination is than 24 MP APS-C at the same size), and I can go somewhat larger (I've tried cropped 24x46" with good results). I wish I had a 44" printer - my Z7 makes my Canon Pro-2000 feel small.
With my lenses (f4 S Nikkors that are pretty good at f4, but sharpen up noticeably at f5.6 - I like zooms for landscapes a long way from the trailhead), I have a choice of f5.6 and f8 for best results - I'll use either f4 or f11 in a pinch. If I got myself a 50mm f1.8 S, I'd add f4 (without reservations), f2.8 and maybe f2 to my list of choices.
In practical use, 46 MP 24x36mm feels like it's got about a third of a stop more before running into diffraction (good at f8) than 24 MP APS-C (which softens slightly by f8) and both have a bit more than 16 MP Micro 43 (softens slightly between f6.3 and f7.1).
Of course, the smaller formats actually have more DOF, because those apertures translate to different DOF on different sensor sizes. If I were willing to accept the print size restriction of a 16 MP camera due to diffraction, I could of course stop the 46 MP camera down to f14 and get the same DOF as the 16 MP camera, but only at a similar resolution (thus a much smaller maximum print size).
I'm not sure that I understand your "DOF effective f-stop" - I would think that stepping back to take in the whole print would mean that more appears "sharp" , not less? Billboards work because they are viewed from a long distance, even though they're not actually sharp? If I'm missing something, it only supports your point more - the calculations below get less practical, not more...
As 24x36" sensors get more resolving, the diffraction limit gets tighter (as BJL says). It also becomes more challenging for lenses, as two things are happening at once. First, the lens has to resolve more and more to keep up with the sensor, and second, lenses are being forced wider than their sweet spot.
A 100 MP sensor would be diffraction limited around f5.6 (only one "good f-stop" on most zooms). Only the finest zooms and good primes will have any chance at all at any stop.
A 200 MP sensor is limited around f4 (and the resolution is so high that only a good stopped-down prime is taking any advantage of it) - Sigma Art, Nikkor S, Sony G, Canon L or better at its best stop MIGHT capture more detail at 200 MP than at 100 MP.
A 400 MP sensor is limited around f2.8 (and the lens had better say "OTUS" on it to have any chance at all - fortunately, the best stop on the Otus range is f2.8 ).
A gigapixel sensor is diffraction limited at f1 or so - WHAT lens resolves a gigapixel at f1? If there's anything at all, it says "Property of the CIA" on it, is permanently mounted on a spy plane, and is impractically heavy for any other use
By the time you reach 400MP at f2.8, the conditions for a photograph become REALLY limiting. ONLY an Otus, ONLY on the finest tripod, ONLY at f2.8. That's really only for art reproduction and the like, since even "all-infinity" landscapes will be limited by atmospheric effects. All-infinity landscapes are rare - even Ansel Adams rarely had no important foreground detail (I can't think of one, although Monolith - The Face of Half Dome and Mount McKinley and Wonder Lake come relatively close).
Even 200 MP becomes so constrained that it severely limits options for using the full detail of the camera. 100 MP is tricky to handle, but it still has a significant usable range.
Where this gets even more interesting is as the sensor size changes... Going towards medium format, we're mostly OK (although the increased coverage requirement makes lenses trickier, that's counterbalanced by the extra leeway from the bigger sensor).
50 MP small medium format is diffraction limited at f11 or so (gives us one extra choice).
100 MP small medium format and 150 MP big medium format (Phase size - close to 645 film) are diffraction limited around f6.3 or f7.1 (getting restricted, but still workable). The next generation of 24x36mm sensors (around 60-65 MP) will be very similar to this, as the pixel size is the same.
Going smaller, we start to hit limits - for a while, the ability to build sharper lenses for smaller formats really helps (Fujinons and Olympus PRO lenses are really sharp). Eventually, the required lenses become absurd - a bigger sensor would be lighter and cheaper.
26 MP APS-C behaves like 150 MP Phase, 100 MP Fuji MF and theoretical 65 MP 24x36" (all are the same size pixels, and limited at f6.3-f7.1). That's still very reasonable since you don't need huge coverage - there are plenty of lenses that will give you that performance, including pretty much any Fujinon and better lenses from other makers.
APS-C in the 32 MP range would be diffraction limited around f5.6.
APS-C in the 50 MP range would be diffraction limited around f4.5. This starts to be tricky for a lot of lenses - consumer zooms may not open that wide, and are certainly being tested (and failing) on their wide-open performance. The best zooms (red-badge Fujinons) possibly have a single non-diffraction-limited aperture that resolves that well, especially in the center of the frame. Really good primes have a few apertures to choose from.
Micro 43 is limited around f6.3 at 16 MP, f5.6 at 20 MP. Olympus' PRO lenses give quite a bit of flexibility with the existing sensors, although the consumer zooms get quite tricky with maximum apertures close to the diffraction limit, while they benefit from stopping down. Even the PRO zooms have a slight problem at the other end - you can't get much bokeh because the widest aperture (fortunately, those lenses are highly usable wide-open) is still equivalent to f5.6 on 24x36mm. The primes open wide enough (and are usable enough wide open) that they aren't a problem.
A hypothetical Micro 43 sensor at 40 MP would be diffraction limited by f4 -the PRO zooms might well still have one good aperture (the f2.8 models almost certainly resolve that well stopped down one stop, while some of the f4s, particularly the new telephoto, might perform well enough wide open). The best primes have a narrow, but usable range on a 40 MP sensor.
As sensor sizes reduce farther, the corner gets tighter...
A 1" sensor at 20 MP (the common current sensor) is diffraction limited at f4 - the long end of your RX100 VI's zoom can't use the full sensor resolution, the wide end has a lot of distortion correction and the middle of the range uses the full resolution only at a single aperture. A 20 MP 1" camera with a good prime would have a couple of full-resolution apertures.
A 2/3" sensor is diffraction limited at f4 at around 12 MP, f2.8 at around 24 MP. It's JUST possible to build a high-resolution (24 MP) compact with a very good prime that has a single aperture with full performance (remember how good that lens has to be to get 24 MP onto the small sensor). Somebody's probably built one once, although B&H doesn't presently have any. The 12 MP zoom cameras in this class might use their full sensor resolution somewhere in the zoom/aperture range if they have really good lenses.
A 1/2.5" sensor (cell phone) is at best diffraction limited at f4 around 6 MP, f2.8 at 12 MP. The sensor in an iPhone XS has an active area of 5.6 x4.2mm (it's a little bigger, but that's the area used to form the image) - it has the same pixel density as a 440MP 24x36mm sensor (e.g. the impossible sensor that is "Otus-only"). Does anyone think that little phone lens has the center performance of an Otus (it only needs center performance, because the chip is tiny)? It may not be diffraction per se (the phone lens is fast enough for 12 MP, although not for the higher-resolution phone chips we're seeing), but the required lens performance is just not realistic for a lens that is built to be tiny.