I would be quite sad to see Pierce go [obviously], but I don't even really see this as debatable. Carmelo has at least as much to offer this season (fitting in with the team is the only possible issue) and that gap is only going to increase as the years go by.
I think Carmelo's defense is also a pretty big issue. As of right now, Carmelo doesn't play any. Since our defense against elite teams like Miami relies so much on team defense, I think this trade would probably result in us not making the Finals this year.
Also, Carmelo has played selfish basketball this year. I'm not sure if he'd have time to adjust to our system, where no player is more important than the other stars on the team.
Is it worth lengthening our window, if it lessens our chances this year? When you're this close to a title, you need to maximize your chances, not focus on the future.