I agree with your basic premise. Outside of certain favorable situational matchups (such as when Pierce has a smaller 3 guarding him), the only guys on this team who have ever been good post players are KG and (I think) JO, but KG is almost exclusively a jump shooter now, and JO's offensive game is nearly nonexistent. And when you look at the recent title winners, you see that interior size is the primary factor: the Celtics had KG and Perk; the Lakers had Odom, Gasol and Bynum; the Mavs had Dirk and Chandler; the Spurs had Duncan (and Robinson as well in '99). Having a solid-to-dominant interior presence is a huge plus in itself, plus it opens up the perimeter guys. If Boston had a legit post presence—just one guy, even—it would make, I believe, a world of difference. Boston would get more high-percentage shots, more easy baskets, and more rebounds, and Pierce, Ray and Pietrus would get a lot more open looks. Unfortunately, there are like 3 good low-post players in the league, and they may all be out of Boston's reach (although that Rondo-for-Gasol idea is awful tempting to me).
Sadly, the low-post game is a dying art form that's nearly gone. Oh, to have another Kevin McHale- or Hakeem Olajuwon-like player! There are some guys playing now who have the strength for a low-post game, but they've got no moves, mostly because they've been able to get by, their entire lives, on brute strength and alley-oops—think Blake Griffin and Dwight Howard. Heck, even Shaq had a limited repertoire, usually just using his immense girth to back guys down and dunk over them. I mean, that's effective, and I wouldn't turn down such a player, but those guys could be so much better if they had actual moves. Just imagine how good Howard would be if, say, he had a good up-and-under move. He'd be unstoppable.
Then again, a lot of the fundamentals are lost on today's players. Paul Pierce rightfully gets a lot of credit for his effective use of head and ball fakes—so why don't we see lots of other players emulating that? We all know how devastating McHale was on the block—so why don't today's tall, lengthy players emulate him?
Perhaps this is the downside of having so many highly athletic players: after all, if you can get out and run and play above the rim, you don't need to be able to do anything other than pass and/or catch an alley-oop. Trouble is, if a running team goes up against a good defense that keeps it from running, they've got no options besides jacking up threes (something else most players today don't do well). It seems like not a lot of guys want to put in the time to develop a multifaceted game that would make them truly dangerous, which I guess is why the league has a lot of players who are fantastic on the break but pretty inept in the half-court.