[SUMMARY: There's been a lot of talk about banking scholarships. 2015 and especially 2016 is not the time to do this. It will not help. However, 2017 and especially 2018 may be the time to do it.]
Okay, so everyone can probably agree that the ideal situation is to have at least 20 recruits each year, thereby leading to a steady supply of juniors and seniors to lead the team on the field. This is the best way to always be in contention for top-tier recruits -- sustained excellence on the field.
So let's assume that one of Harbaugh's goals is to lay a foundation over the next six years for this kind of balance. Is that even possible? I think yes, but it will take some luck -- mainly with the talent in 2015 and 2016 -- these classes need to both stay and be productive.
Act One is make the 2015 and 2016 classes as large as possible. Do what Harbaugh appears to be doing, which is to encourage non-essential redshirts to play their graduate year elsewhere. That is a no-brainer. They've done all they can for 2015, given the short time frame -- fill the spots and hope for the best. Positional needs will have to be filled next year.
Act Two is get the 2017 and 2018 classes down to sustainable numbers, like 25 at most. Fewer if possible. There are 16 redshirts in the 2017 class, and 15 in the 2018 class. A lot of those need to stay around for their graduate year.
Act Three is to get the numbers up to at least 18 for the 2019 class and then at least 20 for the 2020 class. Thereby achieving the above-stated goal, going forward. There are 12 redshirts in the 2019 class -- even without any attrition among those 12 (unlikely), you still need maybe five players from the 2015 class to NOT redshirt. Right now, that seems like a pretty tall order. David is a lock, but who else?
Bottom line is, if the will-likely-play-as-freshman recruits are not coming this year, then Michigan should look for players who can at least contribute on special teams and graduate in four years. There's no harm in doing so. You're going to need those scholarships in 2019, and they are useless in 2017 and 2018.