"Windows Rot" is definitely a thing but it can be cleared out by doing a clean reinstall of the OS. While this can be time consuming, you'd likely be doing it anyway if you got a new machine.
No idea where I'd even find an installer for Windows 7! It does make me wonder whether upgrading it would actually help. But for now it works well enough I'd rather not risk it (the other thing I use it for is some old software that requires a FAT partition for its licensing to work!).