In order to make comebacks, you have to score points. In order to win games, you have to score points. To say that its always all because of their defense, is incredibly far from the truth.
Second, to say Collins has the same team is absurd as well. The team has a lot of young players. As they mature, so will the team. They have clearly matured quite a bit since last year (mainly on defense). Not to mention they added a playmaker on offense that they have not had in the time that Vince Young was at the helm (Chris Johnson). Add to it the new, more effective offensive system run by Mike Heimerdinger, as opposed to Norm Chow's crap. So your original statement is likewise ridiculous.
He isn't playing right now because the team is simply doing well right now with Collins, much like the Patriots were doing when Bledsoe went down and Tom Brady stepped in. Why change it, if it is working?