Had to be done. There are two healthy QBs on the roster going into next year and one will be a true freshman. Sure you can wait and see who else pops up in the portal, there will almost certainly be more talented QBs in there. But this guy wanted to come, and is good enough to help you, so they made the conservative move.
Also, there is no guarantee Coan starts. Even if he does, maybe it is only for the first few games
We know Kelly is very deliberate in playing young players and doesn't do it often. But if Buchner comes in and his throwing motion is back to normal, and he picks up the offense and isn't struggling with the transition to college (Like JJ supposedly was), it will be hard to not play him. His upside is leagues better than the other QBs on the roster.