I don't completely disagree with you, and I do tend to agree with the idea of staying the course, but there are two things I don't agree with you on.
I am not confident AT ALL that Schaub will be back and healthy when the season begins, and I have a feeling that he might miss significant time, despite the rainbows and unicorns spouting from the Texans PR machine.
I also disagree that the season would be a write off if Manning joins the team.
We played a significant part of last season with a fifth round rookie, who had zero exposure to the NFL before last year, and he was able to operate the offense. I do acknowledge that he played in a similar system in college, however.
Yates' college experience notwithstanding, I think Manning and the offensive players could get on the same page before the first game, at least enough to score as much as the team did with Yates, and probably more for the first few games, then we would more than likely see the best offense in team history for 10-11 games, IMO.