You would check up on your backed up seed phrases more frequently than a couple of decades and would identify any wear in the ink or paper before it becomes unreadable.
Well I have made a mistake... I can't check on one of my backups quite easily. Don't ask me why
I will however check once per 2-3 years to see if it's ok! Perhaps I will just make another copy of it no matter what the old paper's situation is.