the keys would otherwise be wasted
I understand this sentiment, and partially agree with it.
On the other hand the number of people who are interested in keeping and using 30+ year old computer systems is quite small, while there are literally millions (billions?) of people who use modern computers every day. I feel certain that the number of people in the subset who want good keyboards to use in the modern world is orders of magnitude greater than the number of people who actually want to use primitive computers.
I think you're right that the subset of people who want a good keyboard is higher than classic computer collectors, but the subset of people who want a good keyboard and are willing to hack one out of an old computer and figure out how to get it to work with their modern computer is smaller than classic computer collectors. But at the same time, and especially for rare systems, that subset is enough to drive the prices on those computers to the moon and make them unattainable for those that want to preserve them.
I personally keep around one old computer, an Atari 800, because it's an excellent games machine for its time and it's fun to see what software was around for a machine released in 1979 (thankfully it has abysmal Stackpole switches that nobody would ever want... though its keycaps are very high quality.) Other people like to collect old computers for their games, as sort of "museum" pieces to relive their youth, or just curiosity pieces to see how far computers have come. If the popularity of channels like LGR and The 8-Bit Guy are anything to go off of, there's a decent market there.
Which makes it feel bad to think about somebody wanting to preserve a piece of the past getting shut out of the computer they want because most of them have been cannibalized for for their keyboards.
I'd draw the line at things like terminals because a)you can't really use them as intended anymore, b)even when they were used, it wasn't for anything fun and c)just saving the keyboards from them (and maybe some parts for other old PCs like their CRTs) is probably more than would've been saved otherwise. For example, I salvaged my F122 from a 3180 terminal where the rest of the unit was in pretty rough shape, and to be fair I tried to sell the rest of the terminal for about two months and there were no takers. The Model F would've been trash if I didn't snag it, instead of just a CRT with an incredible amount of burn in.
Likewise for any old computer that was on the brink of being e-waste otherwise. Better to downcycle than throw away.