Data decays on floppies at rest but... If I manage to read say a 5"1/4 floppy from my Commodore 64 correctly and copy it to another, NIUB (New In Unopened Box) but 30 years old floppy, will that new copy last for decades again?

Or is the medium itself damaged by time passing?

I'm asking because during Covid I dug out my old Commodore 64 and managed to read a few disks and created a copy of some that were still working.

It's a bit of both, but I think the bigger issue (at least in my experience) is the magnetic flux pattern, especially if you've got new-old-stock media that hasn't been written to much or physically damaged. If you successfully remaster the old floppy to a new one in good condition, you ought to get a good many years out of the new disk. Of course, it would also be a good time to image that floppy and store it somewhere else.

On the other hand, there are many good disk drive emulators for the Commodore 64 now and these can be had for fairly cheaply (like a SD2IEC with a Epyx FastLoad combination), which will avoid the whole problem. I still use floppies with my 128, but I also push disk images and programs to it with a 1541-Ultimate.