Anything less than a significant upgrade (a proven winner) gives us a lesser chance to be successful next year IMO. I would have not done the extension and told Kubiak, this year is for all the marbles, it's a contract year for you and this team has to get it done or it's game over.
I agree with most everything you said, except how you define definite upgrade.
The main reason I want to keep Kubiak, is that I believe he has built a Helluva team, in a short amount of time. While Cowher may be an upgrade at instilling a certain attitude into a team...... I don't think his in game decisions are a "definite" upgrade. I also don't think he could have built this team as fast as Kubiak (yes, I said as fast) has, so I would quesion how long this team would be successful in the years past 2010.
I believe in my heart-of-hearts, if Kubiak was first given this team in the state it is now, you would have seen that "instant" success "everyone" is looking for.
I can not say with certainty that any other coach would have been able to do what Kubiak has done if they were given this franchise in 2006. Not Payton, definitely not Cowher, not Parcells, no one. For all you literary challenged, that is not to say that Payton or Parcells couldn't have done better....
But for all the people who say the right way to build a team is primarily through the draft, this is what you get... being "smart" in FA... this is what you get.