Nicely summarized, Keith. My experience parallels yours: I have used everything from a 4 by 5 Speed Graphic (for weddings when I was just in my late teens) to medium format ( Mamiyaflex, Hasselblad), and Leica M's. Now it is a Canon 5D2 and M9 and MP. I recently purchased new 35 and 50 Summicrons, but until then I was using the same but built in 1962 and 1967, still have them – I'll grudgingly admit to prefer the new lenses. I do quite a bit of work documenting art exhibitions and copying artists's works for catalogues and their websites. When I went 'digital' I switched from Hasselblads to the Canon as I could not afford a medium format digital back. That works fine. And now I find myself using the MP, which has LV and an EVF, for that work; bonus: no need for a cumbersome emote release, any cable release fits!
I just viewed the videos with Kevin and Charle Cramer and found his recommendation for the use of a viewing card is interesting (I have used those in photography workshops and seminars). With an M9 or MP, you have a built-in viewing card, from 28 to 135 a the flick of a lever. You don't even need the lever, you just eventually learn instinctively what the field of view will be even before raising the camera to your eye.
No digital camera will be useful in 50 years. Will a Leica be useful longer than other cameras? Maybe not. However, there appears to be a market for used M8's and it also appears that many M8 owners simply keep using their camera. That camera is getting quite long in the tooth in digital years but people keep making images with those. My first dSLR, a 5D, lasted one year before being replaced by the newer 5D2.