I agree that it would make more sense to just try to target him in free agency, but he's a restricted free agent... so Philly probably matches whatever offer he gets. They have to spend the money somehow and they could just try flipping him at the 2018 trade deadline to one of the teams that had interest in free agency.
Colangelo already said hes uncomfortable going into this season with all the bigs. The last thing he wants to do is going into NEXT season with all the bigs.
It's hard to say which of the bigs they should keep, though. It probably makes the most sense for them to keep them for the time being and experiment with a variety of lineups. Whatever decision they make could impact them for decades since any one of those center prospects might have all-star selections in their future. Unless they are taking back an established star or someone they believe has a chance to be a star, it doesn't make much sense for them to trade any of those guys.
If things have escalated and Noel is now demanding a trade, that changes things.
Normally they probably trade the guy they think has the least potential.
Like when OKC dumped Jeff Green, he was the weakest of their top young guys.
But as you noted there are concerns about fit - and also salary.
Under the current CBA players with 6 years experience or less can get a max of 25% of the cap.
Simmons will likely get a max deal regardless of how he performs. Too much hype and expectation to risk losing him. That leaves ~75% of the cap for Embiid, Okafor, Noel, Saric, and of course all the guards and small forwards they'll eventually need to acquire to build a competitive team.
Once you have Simmons, how much of that 75% are you willing to commit to the other big men on the team? I can't imagine more than one of those other bigs getting a max. They would end up like the Drummond/Monroe/Smith Pistons of a couple years back.
They might end up trading one of their top-3 players due to position, much like when OKC traded Harden instead of dumping Ibaka or Perkins.
Hopefully our talent evaluation is better than theirs and Danny can get his man, unlike the Green trade when OKC allegedly pulled Harden off the table at the last minute and we got Uncle Jeff instead.