Class D is not digital and there are no "digital" parts in the amps. Class D uses chips that are easily attainable and will be around as long as parts used in Class A amps. Here is the rub......if your class D module breaks then it is usually replaced by another module. They are designed to last years and years and they are inexpensive enough that they can be replaced relatively cheaply. However, if you paid $2000 for a class D amp today and one of its modules died 10 years from now would you even have it replaced for $300 or would you rather get the latest $500 amp that sounds even better.....they will get better and better and cheaper and cheaper. This is, of course, the same with class A/B amps. Imagine buying a $20,000 class A/B amp today and it goes dead in 10 years.......by then you could probably get a $2000 amp that sounds better.....so just have the old amp fixed and try and sell it to someone. We are reaching the end of the "You have to pay thousands and thousands of dollars to get great sound game"
Now if you have an amp designed years ago by someone now dead and there are not any schematics for it (for instance Spectron Audio....makers of class D amps from 20 years ago)........then good luck trying to get it fixed. So, just junk it and buy todays even better sounding class D or class A/B amp for less than what you paid for the Spectron......not really a big deal unless you are really cash tight.