Uh...excuse me? Have you ever seen Nerlens Noel play? Or Al Horford? Horford is more of an outside threat, 3pt shooter type of big now at this point in his career, while Noel is strictly a rim protector and plays closer to the basket. If Noel comes here, they play side by side and actually fit quite well together I would like to think.
Yea, I have. Noel is a useful defensive center who didn't translate nearly as well to defending the perimeter on a regular basis as Philly hoped and isn't good for much on offense aside from picks and rim-runs. Horford at this point in his career, on the other side of 30, is best-suited to playing center on defense as well, as his wheels on the perimeter are only going to get worse as time goes on and he provides pretty good rim protection in his own right. On offense, Horford is
also best-suited as your primary screener, where he's much more dangerous than Noel (if less highlight-y) because he can do all three of pop out for a jumper, roll and finish at the rim, or make the second pass to an open player based on what the defense gives him. Sure, you can turn Horford into a poor man's Ryan Anderson on offense, but it's a waste.
Having Noel on the roster, giving you the option to go big on teams when appropriate, give Horford stretches to rest on offense by camping him in the corner, and improving your rim protection by a bunch when Horford is off the floor is definitely a nice luxury to have, but it's not an absolute necessity, which is the point: this isn't some staring contest where Philly needs to move a big and Boston needs to get one of those bigs - Boston's fine as-is, and none of Philly's bigs put them over the top.
And while yes, they were a top defensive team last year, that was without a true rim protector. And no doubt there were numerous times last year that they had bad defensive breakdowns. The goal in the East is always to beat Lebron. Lebron's calling card is getting to the rim. You aren't gonna beat the Cavs without rim protection, as we've seen the last couple of years.
Nerlens is fine, but he doesn't make the C's better than Cleveland this coming year and retaining him past then gets murky if Boston's able to pull off any of their primary plans. If they're matching a max contract it probably means the summer didn't go their way, but there's also no reason that simply being the team with whom he signs a big offer sheet next summer without giving anything up but money. That's a kind of guy you don't mind giving up some extra pieces for, but there isn't much sense in giving up current "core" guys who have more years on their cheap deals, and you certainly can't move a Brooklyn pick for anything centered on Noel.