RE system rot, it seems to me a soon as you have an permanent installation, there is the possibility of files getting corrupted either via hardware failure, incorrect shutdowns, atmospheric neutrons, newer versions getting overwritten by obsolete ones and so on. In short, anything at all that might alter files in any way at all, including unintentional deleting.
Entropy exists, it is the most fundamental universal law. Data can only go one way. You can reduce the probability of data corruption but it is in principle impossible to stop it.
It is of course possible that the probability is so low that it can be safely ignored.
But it certainly is also a function of how much a setup is fiddled with. On my work winXP, I leave it alone and it has worked well for a number of years now. At home, with me trying out software and getting rid of it, my system inevitably deteriorated. Performance also deteriorated with installation of more and more software as many pieces of s/w leave their own pre-loaders, accelerators, upgrade checkers etc etc littered in the OS.