The team that trades for CP3 now will be able to offer him an extra year and $20+ million more dollars than any other team (using Bird rights). That's why you trade for him now. Do you really think he's going to leave that kind of money sitting on the table? Especially when Boston will have the cap room for another high salary/max player?
And of course CP3 won't sign an extension. He wants a Bird rights max contract, not an extension.
Boston will not have the cap room to offer another max player not with Pierce at 17 million, Paul at 20 million, and all the other cap holds and/or lower dollar players on the roster (including Bradley and Johnson).
Assuming the trade took Jeff Green and one of our 2012 picks or current young guys with Rondo, and we didn't sign anyone to a big multi-year deal in the meantime, we'd have ~$18 mil to offer a free agent after resigning Paul to the max. Dwight Howard would have to turn down Orlando's bigger hometown offer to join us, but he'd have to do that anyway if he wants to leave Orlando and they won't trade him.
And don't forget the amnesty card. Pierce could be swept clean off the books if he's not carrying his weight.
That's not to say I'm crazy about the idea.
I think Rondo is pretty well-respected around the league: he's got a ring, two all-star games under his belt and a lot of exposure from 4 consecutive significant playoff runs. He's certainly less of a draw than CP3 or Deron Williams, but he also costs a lot less, and it's not like he's chopped liver to begin with.
Rondo, Pierce, a quartet of cheap young players (Bradley, JJ, our two 2012 picks), in addition to the well-regarded Jeff Green and/or cheaply retained twilight versions of Ray and KG should be very attractive to a free agent looking for a more competitive environment. We won't be in a rebuilding mode (in this scenario the Big 3 will all be on hand as veteran support) and we won't have any albatross contracts (Rondo's a steal, Pierce can be amnestied if he falls off, Ray and KG would be on smaller, short-term deals). Not a bad set-up.