KG's contract expires after 2011-12. The "possible" free agents that summer include Carmelo, Dwight Howard, Kevin Durant. Now obviously I'd expect their current teams to try to get them to sign extensions, but you never know. It's quite a few years from then and anything can happen. When it comes to franchises like the Celtics and the Lakers, fortuitous things happen like getting KG or Gasol rather easily. With the amount of cap space they'd have, it's absolutely conceivable a superstar who's frustrated with his current team could bail and go to Boston. And again, trades can happen or getting a franchise type player thru the draft can happen. But Boston won't be building around Rondo as their next franchise player.
The point with Magic's and Worthy's contracts is to show that financially the Celts can make moves once Pierce and KG are off the books, as West waited for there was room to make moves financially to acquire a franchise player.
Again, I'm not saying Boston will remain contenders. I'm saying if they are able to, it won't be by them building around Rondo and Perkins as the franchise pieces.