Why should they have tied up long-term money in guys who aren't good enough to make a significant impact?
Hawes is averaging 13 points, 8 rebs and a block a game at just 25 years old, which means he's probably not even in his prime yet and is better than any center not-named KG that Boston has had since The Chief. As a starter in Philly, Evan Turner was putting up 17 points, 6 rebounds, 3 assists and 1 steal a game. Those are pretty much the same stats as Michael Carter-Williams is putting up. He's 22. Turner is 25.
Again, there's very little chance that any or all of Philly's 2nd round picks will be as good as Hawes or Turner. And when Luol Deng walks away from Cleveland after this season, you'll understand why Chicago only got 2nd round picks for him.
What could Philly do? Keep Turner and Hawes. Add Noel and two more lottery picks this year. They'd still be very young. They'd still have salary cap room. They'd also be significantly better than what Philly put on the floor this year or what they'll run out there next season. With MCW, they'd have 6 lottery picks on their roster. In other words, they'd have next year exactly what you're calling for them to spend 2 or 3 more years building up toward.
And again, they've left themselves with one and only one way forward. They can't attract any decent free agents. They don't have any assets worth trading for, unless it's a team just looking to unload a high-priced player they don't want to resign who will then flee Philly as soon as his contract is up. If they don't get a franchise player in the draft this year or next, they're going to end up exactly where you're so afraid of them being. A mid-level team without any chance of winning a title and little to no way of changing that.
Boston has more 1st round picks than Philly. They have more tradeable assets than Philly. And they can have huge salary cap room after next season. And they've done all that without reducing their talent level to a joke.
Philly turning Holiday into Noel and another 1st round pick was a defensible blow it up move. Unloading rotation caliber players for 10 cents on the dollar when you already suck enough to get into the lottery isn't. Should Ainge have traded Al Jefferson for some 2nd round picks in hopes of tanking his way into Durant or Oden? What if he'd done that and wound up with Oden? Where would that have left Boston?
Look, if Philly had been fighting for that last playoff spot, dumping Turner and Hawes would have made some cruel sense. There's no logic is downgrading your team when you're already lottery bound. The minute increase in your chance of getting a higher pick doesn't outweigh the talent void you're creating for yourself.
This is the equation. MCW+Turner+Hawes+Noel+2 lottery picks > MCW+Noel+2 lottery picks+Five 2nd round picks.
Mike