Well, you used to know, to some extent. All you had to look at was how many were in your recruiting class. Now, do you need to know how many were in the recruiting class, and guess how many transfers are coming over.
Last year our school had a recruiting class of 14. Five are left. Our school has taken lots of transfers.
I think it depends on the programs philosophy. There are programs that recruit on a 3 year cycle because of the draft. There are many, IMO, who aren't good at roster management. I think it's special skill not just reserved for P5 coaches.
The majority of the top programs do not want players staying forever so recruiting is a large part of the craziness.
So if a player looks like they may not get drafted in 3 years as previously thought, they may part ways or the player leaves thinking the grass is greener and that's ok.
A program will never have all of their committs graduate. But a good program will know how to bring their committs along fairly and slowly. Of course the more talented the prospect the less time that he will sit on the bench. It's not all about, IMO, bringing in more because you only need half. It's about bringing in the RIGHT guys for your program. You don't have to be a top P5 HC to achieve that goal.
It's also about the committs making the right choices as well.
JMO