Well, my original question was pertaining to the last 50 years. Using 100 year old rounds was outside that scope. I curious just how many in the last 50 years had come and gone, I can think of a few but not that many (25NAA, 32NAA, maybe some "Ruger only" rounds?). Have to figure, niche round will always pop up, and occasionally they might even stick around.And that matters how? If you cannot find it, you cannot find it.
A Ford Model T is also a century old. Can I go buy one reasonably easily or maintain it easily or use it easily? Probably not.
I agree with you about issues finding ammo. But straight wall cartridges are quite easy to reload, so buy a few boxes of defense loads, shoot sparingly, buy more when on sale. Reload the empties to save a buck. Done. A number of shooters do that with commonly available rounds.