This is already a thing, you can stack medical redshirts infinitely.
Jared Folks played at Temple and ETSU for a combined 8 years. One year was a freebie from COVID, then he also had three redshirt years.
Sure, but if the 5 year rule goes, you could do it without medical need. Just sit out extra years or “unenroll” for a year or go to juco and play or whatever.
Imagine you get a QB but you have a great starter ahead of him for 3 years. You could have your backup QB on your team for 3 years practicing and learning the offense, and then as a “senior” he finally plays and has all 4 years of eligibility left.
Even if the waiver is pretty specific to juco years, you could still have guys go spend a few years developing and then come back up or something. With NIL, a team could pay a kid to go play juco ball somewhere else then come back with full 4 years.
I mostly worry the whole 4 years of eligibility rule eventually get blown up and then college truly just becomes minor league pro sports.