That's certainly a potential issue, although even collectors who use their vintage machines somewhat regularly likely don't put too many hours on the thing. Some parts will fail and fewer and fewer working examples of any given machine will be around as time goes on, but I don't think it's unrealistic to think that a few will still be in working condition 100+ years into the future if taken care of. Given the explosive progress in technology, I wouldn't be the least bit shocked if most of this stuff is eventually replicated in FPGAs or some future technology. Plenty of people already devote time to doing such things just because they can, and I know there is at least one mostly functional Mac Plus FPGA clone. While it's true that Macs were notoriously poorly documented, many parts have been reverse engineered and enough is known to allow fully functional emulators to be written. I have little doubt that just about any ASIC in one of these machines could be reverse engineered well enough to implement it in programmable logic if the need ever arises.