I hear this a lot but there are plenty of examples of computers lasting for decades. The usefulness of these cameras might come into question, but they are not necessarily going to die within 5-10 year as people always seem to think. I truly think there will be digital cameras that will be hip to use for retro effect. It happens in everything else, why wouldn't it in cameras?
I'd say it's too early to tell. The earliest microprocessors ever created are a lot younger than a typical Leica M3. People are still using Leica II models, but there is no microprocessor based computer that old, so we can't really compare them.
There were of course computers before microprocessors, but they were based on far simpler parts which can comparatively easily be recreated right now. It's relatively simple to make a resistor, a diode, a transistor, a capacitor etc, but once (say) a MIPS R12000 processor goes out of production, it'll take a minor miracle to get that production running again. Simpler, earlier processors could be emulated on an FPGA or the the like, but you'd still have to adapt the rest of the computer around it to accept the new processor.
Then we have the fairly obvious fact that digital camera like the M9 are not based on simple computers, they are microprocessor based, and those parts are either in production or they are not. It's not like a simple component like a capacitor, where a replacement will be easy to source. The fact is that in 50 years, the microprocessor(s) in side a Leica M9 will almost certainly have been out of production for decades.
Even if you could isolate the broken part, find a replacement, the chances of you being able to remove the old part and solder in a new one are pretty remote. The circuit boards were not made to be fixed/maintained any more than an iPhone was.
