Well, Barbay is not a really good coach. At least not right now. We would have at least shown improvement if he were and we would know what the end goal is going to be.
But, it's also not really true that really good coaches change their system to match their personnel. Most really good coaches, particularly offensive coordinators, run their system hell or high water and all they can do is try to adjust the plays they install and call to help. Leach is a good coach and basically took the position that any short term gain from modifying the air raid to match his personnel would just delay them from being able to implement it well. Mullen tweaked his playcalling for Tyler Russell and Chris Leak, and basically just ran a less effective version of his offense, although having stellar skill position talent allowed the Chris Leak offense to work fine, and having Tebow to handle short yardage allowed it to run basically at peak level.