Kelly has never had a "successful" qb because the the whole design of the offense is a fail. Book's "reason" for a lack of real success is a combination of lack of arm strength as well as happy feet. Wimbush's was the "yips." There is always a fatal flaw with whoever is the qb that gets worse in the qb's 2nd year when you think it would get better. There are few perfect qbs but Kelly wouldn't know what to do with one if he stumbled across one. As it is, the offense could be better if Kelly could figure out what Book is good at rather than Kelly trying to make Book good at what Kelly wants him to be good at. The sad thing to think about is that Book will likely be the qb in 2020 when he should probably have been surpassed by more talented qbs but he will end up being the 2020 version of Chris Finke or Joe Schmidt.
The preferred passing routes take way too long to develop, much the same as his favorite running play. There is virtually no counter action. Please identify the last successful screen to a rb. For some reason, the offensive line always seems to underachieve given the level of talent it has. Etc, etc, etc. The problem isn't the quality of the players.