Fred: I'm sure you know all this already. Was objecting strongly to the made up term 'bit rot', which seems to be a handy catch-all for "this chip contained bits. now the bits don't read correctly. what happened to it? bit rot!". :P
It was mainly aimed at the (still large number of) caring Amiga owners that are afraid their hardware will "decay". Having repaired arcade PCBs that have received inhuman treatment, I'd say everything is fixable.
The exception is proprietary chips whose function cannot be determined (say, by one who has programmed them or dissected them physically) or replicated (nobody still around who can describe the exact function or lost designs).
So I just wanted to say to them, "DON'T PANIC"
Floppies can be emulated or RADed, and screens displaying RGB in 2D will be around for a good while.
Timing seems to be at the essence of this. As long as replacement parts (or boards) mimic the timing of custom logic exactly, repairing original Amigas will last as long as the physical substrate and copper does.
Charlie: Let's get practical. Are you predicting actual decay of printed circuits in a (~) vacuum in a chip that never overheats or gets a voltage spike in, say a couple centuries? Or what do you mean?