Historically I wouldn't refer to it as "bit rot", but generally "registry bloat" with a combination older, no longer used .DLL's hanging around, rather than being removed on software uninstallation or upgrade.

In the past a good "registry cleaner" would help - but those are no longer reliable with newer versions of Windows - there are many virtual entries that get cleaned-up by overly aggressive utilities.

> I had 1 same Windows 10 install running on desktop for 8 years as primary personal PC

I actually have a desktop still running that got a launch party host Windows 7 Steve Ballmer edition install that's just been upgraded as time has gone on. Very much a Ship of Theseus machine but technically only ever migrated the OS image around, never reinstalled. That's 17 years of a Windows install so far, and its perfectly fine. That one install has made it through multiple motherboards and OS upgrades. It'll end up dying and being replaced once I get too uncomfortable with 10 EoL, this board is still useful to me but it doesn't have a TPM so Windows is dead to this machine.