The thing is, SLR is a technology that's reaching the end of its service life, to be mostly replaced over the next decade by mirrorless systems (of whatever size).
The value for current SLR users is to be able to move into a new system that's able to replace, not supplement, their current systems, while, for those who do not currently own a full-frame system, it is in the ability to move straight into a full-capability, future-proofed mirrorless system rather than having to temporarily buy into a system that's likely to become dated and suffer from reduced manufacturer support in the near future. That calls for f/2.8 zooms and fast primes which can entirely replace their SLR counterparts, not slow lenses.
Mirrorless needs to be a full-capability system, first and foremost. Those buying into the system shouldn't also need an SLR system if they want the full gamut of capabilities. They need to be able to get it all with just a mirrorless system. The small lenses can come later.
Future proof might be the most overused word in photography. Red saw to that a decade ago.
Your scenarios all seem to suggest all this switching is going to happen the day the cameras are released, and that it’s going to happen all at once, and that all the systems are defined by what’s available when the first piece hits the shelves. None of these is true. People rarely dump an entire system and pick up another with no overlap. That takes time. And during that time, all the manufacturers will be releasing new lenses, and new bodies as well if the trial is long enough.
You talk as if Leica/Sigma/Panny will have a whole line of professional lenses available when the body hits stores. And that that invalidates all the other manufacturers’ strategies. Neither of those is true either.
By the time the Panny camera hits stores, Nikon will have more S lenses available. Canon will have more R lenses available. Depending on what the delivery timeframe turns out to be, Nikon’s trinity of 2.8 zooms you’re so concerned about may be available in the S line. And who knows what’s on Canon’s roadmap?
Meanwhile, there are a few dozen Nikkors that will that will autofocus well with the FTZ adapter, and more manual focus lenses. There are a few dozen Leica manual lenses that will work with the M-L adapter, and a few autofocus lenses. And there are a few dozen Canon lenses that will focus well via the R adapters. The adapters work. (Well from all reports.) And that means real photographers have real choices.
I’m very happy with the direction Nikon has mapped out. I’m intrigued by the L/S/P alliance, and interested in the Panny FF announcement (but not because of f2.8 zooms). And I’m sure of only one thing regarding the future—the evolution from DSLR to mirrorless cameras will be a lot more chaotic than the writing on this forum suggests. Right now, I own four camera systems, and have another on order, and I sometimes rent stuff. That may be above the mean, but having different cameras for different purposes is not uncommon.
18 months from now, I expect we’ll have three new and pretty interesting camera systems that will serve the vast majority of serious users with emphasis on different things. They will all have very good, but still basic, lens lineups. Sony will still have more native lenses, and that will matter to those for whom those more exotic lenses are important. Nikon will not have gone under because it offered f/1.8 primes, and Canon will not have ridden to world dominance on the back of a two-kilo f/2.0 zoom. And I might still own both the Sony and the NikonZ systems. Or one may have been gone to eBay. Or maybe both will have gone and been replaced by some L/S/P kit. What will determine that? It won’t be the native lenses; I’m confident all three systems will have a pretty fair selection by then, with more on the way. It’ll be the responsiveness of firmware updates, the ergonomics and UI, and how well the bodies and their various adapters work with third party, mostly manual-focus lenses, including vintage Cine lenses and specialty technical lenses that are important to me. That’s where “future proofing” lies (if it lies anywhere at all). It lies in the firmware, the usability, and the compatibility with legacy glass that provides unique looks. All of them will have very good native optics in the usual specs, along with a few showpiece lenses.
