Thomas and Decker are nice and all, but who do they have beyond that? Also, Thomas is raw as fuck coming out of that offense at Ga Tech. Then he started to learn a pro offense in Denver, then they ran whatever the fuck they were running with Tebow, and now he's going to have to go back to learning how to run a normal offense with Manning. Let's not act like that transition is going to be seamless.
Plus, those guys really benefited from the style of offense under Tebow. Teams loaded the box to stop the option bullshit, and Decker and Thomas often had single coverage on the outside with one safety over the top or no safety help. Now with Manning and a normal system, they won't have that same luxury nearly as often.
I don't see the running game being as effective either. Without the threat of Tebow and a gimmick offense, McGahee is basically the 3.8 ypc guy he was in Baltimore in 2010.
The AFC West race is going to be really interesting and not nearly the cakewalk people are assuming.