Let me just first start out by saying that Rondo shouldn't be called untouchable. If an absolutely irresistible offer comes along, then of course trade Rondo. In every other case, don't trade him. And that's what this post addresses.
Obviously, I think it's self-explanatory why we wouldn't want to trade Rondo, but I think what will need explanation is why I think we should keep Rondo even if we rebuild. So:
Pretend we trade Rondo to cover all our bases and ensure we have the best possible chance to get a total franchise changer. For the rest of the post, I will simply say refer to Wiggins out of convenience, although I'm well aware that there are more potential game changers such as Randle and Harrison. Even with increased odds, it's still no guarantee, but for the sake of argument, let's say the best scenario pans out and we nab our Wiggins. What the heck does Ainge do now? He's completely gutted the roster of any sort of talent; what free agent in their right mind says yes to a completely torn down team that features one really promising player, but otherwise a bunch of role players? Even after OKC drafted Durant, they needed to follow him up with Westbrook before they could mold into a contender. What's to guarantee that such a perfect situation falls into our lap? And don't give me any of that Nets-will-suck-soon BS because fact is, that is no where near being a guarantee. With their deep wallets, they can and probably will reload after each of their massive contracts expires. Considering all this, is it truly worth trading Rondo and getting our Wiggins?
Instead, wouldn't it be far smarter to keep Rondo? Sure, we go from winning 20-25 games to winning 25-30. But that doesn't instantly torpedo our chances at a top 5 lottery pick. But for the sake of argument, since we considered the best possible scenario above, let's look at our worst. Eliminate any chances of getting a top 5 lottery pick. However, we'd still practically be a lock for a top 10. That's still phenomenal considering the 2014 draft class. While we won't nab our Wiggins, our chances at a very good player is still high. We don't get our Lebron/Durant, but we can still get our Lillard/Cousins. And you know what? Given Ainge's draft history, I certainly wouldn't rule out a George/Curry...let's not forget Kobe was a 13th overall pick, and the 2014 draft might just be the best one since 1996.
What's so good about not trading Rondo is that now, we have a guy who instantly makes his teammates better, and undeniably helps the growth and development of young players. For all the talk that he's a "bad teammate", time and time again, the young Celtics always point to Rondo as being a stable veteran presence. Let's not forget that Bradley was looking to be a bust after his rookie year, but according to Bradley, Rondo pushed him on, and now Bradley's a defensive stud. Furthermore, free agents wouldn't instantly scoff at Boston since we wouldn't exactly be a completely scrapped team with a rookie as our best player.
EDIT: I've also seen a lot of Rondo hyperfans think the C's can actually compete for the playoffs if we keep Rondo. I definitely disagree, but if that would happen, that'd be great. Rondo proves he can be that superstar caliber player to build around, the rest of the league will take notice, and then there'd be absolutely no need to debate whether to retool around Rondo or rebuild.