Mike,
many parts aren't available yet, like i.e. VIC chip... there are tons of machines broken without chance to be fixed because of lack of components, so it would be fantastic if somebody (some chinese firm? ;-) ) could develope a replacement....
The SID, the VIC, the PLA and many other chips have been reverse-engineered and can be emulated. Jeri Ellsworth did it in the C64-DTV. As far as I understand, she was well underway to emulate the Amiga 500 chipset in an FPGA too, but the project got canceled.
The bottom line is: Nothing lasts forever. But you should wonder what really is worthy of preserving? Most people now work with computers, smartphones, TV's that are really computers with tuners, etc. so you're never going to be able to re-create the experience from the 1980s where you had never ever seen a computer before and then all of a sudden there was this box in your house that could do so much, if only you knew how to use it
Personally, I think the most important thing is to keep the information about the hardware alive, as well as the software that runs on it. It may not always be possible in the future to keep the actual hardware alive, but as long as the information is available that can be used to rebuild that hardware somehow, or to emulate it on some other hardware, it will be possible to run the software too. So thanks to websites such as Zimmers and Bombjack, who take it upon themselves to keep this information alive, the history of those old computers is safe, even if they run the risk of getting shut down every day because of the retarded copyright laws in the United States.
But it will be a long time before all the old hardware will turn to dust and will be unrepairable. Most of the standard chips (like TTL chips) will be available for a number of years and the custom chips like the PLA, SID and VIC are so well understood that it doesn't really matter if we run out: they can be emulated if there's no system available from which a working chip can be removed.
It's unavoidable that if you have a broken computer from the 1970s or 1980s, some ICs are already not available anymore, except in other machines of the same type. Eventually other chips will have the same thing happen to them. Eventually there will just be no other choice but to emulate most, if not all of the computer. But it will take so long before that happens, that I doubt that anyone will care when the only way to fix a C64 is to replace it with another C64 or an emulator, as long as they can run the old software and experience the "look and feel". If anything, the keyboard is the most important hardware to keep alive because it's probably the only thing that can't be emulated
===Jac