if the QB's play well he's gone (to a better job) and if they play poorly he's gone.
But isn't this true of any assistant coach? Any of them could get a great offer after doing a great job here. Conversely, if they don't do well....they could be replaced or at least moved into another position. So I'm not sure what your point might be here, in how this is different.
Personally, I like the hire. He's coming with NFL experience, just as Pasqualoni and DeLeone did. He had a chance to work with a very good offensive mind in Martz for three seasons. In fact, Martz thought so much of him, he brought Day along with him to Chicago when he left SF.
This will also be his first opportunity to the head QB coach at the BCS level. I'm glad he's up for that challenge. He's younger too, which adds an element most of us were seeking as well. Hopefully his recruiting ability will be another positive...only time will tell!