No guarantee that players pan out. With the current QB situation, I'm almost inclined to say take two this year and two next year ('15 is stacked!). You'll most certainly get a feel for who can hack it by the end of '16 season. The ones who can will stick around, the ones who can't will probably transfer.
At any rate, ND re-stocks the shelves and puts itself in the driver's seat to evaluate the potential candidates and play those who fit the bill. The ones who don't, thanks for applying*. CFB is becoming (and already has at most schools) a business. If ND wants to compete for a championship year after year, they need to continue taking chances on getting the best players and keep them around (or at the very least have another highly rated player waiting in the wings to replace the ones who leave).
This plays into my minimal over-signing plan. These recent transfers aren't the first and certainly aren't the last. It's vital to maintaining a level of elite power long term to be able to lose one guy and instantly plug in another player w/o fall off. The QB position is no different. I have faith in this staff that they'll make the right decisions and coach up the players to truly be the "Next man in" without the team suffering any set backs.
*I'm not suggesting those players who fall into reserve roles be cut. I'm simply implying that when a roster becomes so saturated with elite talent, the ones who get buried on the depth chart have a higher tendency to transfer. Bringing in four QBs over the next two years will almost certainly result in at least one transfer. However, that means there's at least one or two who have stepped up and earned PT. I'm good with that scenario.