Take out the whole concept of years. The final two teams this year played a total of 16 games. So why not use that as a benchmark. 16 games of eligibility over 4 years would be 64 games. So give everyone 64 games of eligibility that they can use over any period of time. Then it would be up to the schools and conferences to incentivize the players to keep playing in every game each year. Or they could selectively manage the roster and sit out key players on purpose for cupcake games and get more time out of them. So if someone wants to stay in college and play for 8 years, then they can, but they would be limited to playing in just 8 games each year. And eventually they will get bored or too old/slow/weak and someone younger will take their place, just like in the NFL.