Yes. I think it all boils down to one question: Does the average amount of time athletes occupy roster spots in the college sports system increase now that they can play 5 years?What you are missing is that there will be a bunch of new 5th year players available, and more as time goes on because players won't redshirt. Thus, more good frosh will be playing. So, those two things will be a double negative whammy for high school recruits, as well as older marginal players.
There is only so much playing time in the system. There are only so many scholarships. There now will be more older, seasoned players playing and taking up playing time and scholarships. Every good 4th year player is now eligible to play a 5th year. Every good frosh is now available and will add to the number of frosh playing. The only offsets are 5th years who have gotten hurt or who decided not to play a 5th year. Those who redshirt already stay for a 5th year, so why wouldn't they play their frosh year and stay for a 5th year?
The forecast/predicting of rosters will now change because of what I said in my first paragraph.
If yes, annual turnover falls and vacancies decrease.
If no, annual turnover and vacancies remain the same.
I would think it's a safe assumption that, on average, in the whole of college sports, the average time players play would increase if the new default became 5 years instead of 4.