I don't think anyone is truly untouchable, and if the C's could turn Perkins into the #2 pick in the draft, I think you would have to look at it.
Perkins is incredibly valuable, and has a great contract, however, he also has a bum shoulder, and may not be a great building-block once the big 3 are gone, and he is not as much of a bargain anymore (I expect him to be looking for a new contract starting at a minimum of $7-8 million, and there is a good chance, he will be making $10+ million per year on his next deal).
To me, the key will be what the diagnosis of his shoulder is, but if they foresee the problems to continue, if not getting worse, it could make a lot of sense to move him now, if they can get such good value for him as the #2 pick. They could then either try to trade down a bit, or use that pick on someone like Harden, Trade Ray Allen for Chandler and Antonio Daniels, and then use the trade exception from trading Perk and Walker just for the pick to get someone like DeShawn Stevenson. And then of course filling out the roster with a big man like Sheed or McDyess, and maybe an vet minimum wing like Dahntay Jones, and resign Powe to a minimum contract.
Then you have a roster like:
Rondo/Daniels/House
Stevenson/Harden/House
Pierce/Jones/Harden
KG/Sheed/Powe/Scal
Chandler/Sheed
And you also have a young core of Rondo, Harden, Powe, and Chandler to build around as the Big 3 fade away.
That roster would be significantly better defensively than last years, and possibly the year before, and while they lose a little offense from the first unit (although Chandler is an offensive upgrade over Perkins), the second unit is vastly superior with Sheed, Daniels, and Harden.