Covington in his career has performed as well as Crowder. Now obviously thus far this year Covington can't hit the broad side of a barn and Crowder has been on fire, but those are both career anomalies. Covington is the better rebounder and they are comparable passers (in their careers, though Crowder is better this year). Both are + defenders and can defend multiple positions. And yeah Crowder has a great contract, but Covington's is better. They are both 26 and are pretty firmly established as 3 and D players. That is their end game.
Noel has incredible defensive talent. Could easily be a Deandre Jordan type defensive player, though Noel can hit foul shots and doesn't rebound as well as Jordan (still would be by far the best rebounder on Boston though). I could easily see him being a defensive anchor on a championship team, but could also see his attitude and injury history having him out of the league in the near future. Noel is still just 22.
Saric as a rookie has shown flashes of greatness but also does the typical rookie things. I think he has by far the most top end potential of any of the 4 players especially if his 37% from three isn't a fluke to start his career. He could be a real legit star.
If Philly called and offered Noel, Saric, and Covington for basically Crowder and Ainge turned that down he should be fired on the spot.
I like Covington better as a 4 than as a 3, whereas I feel good about Crowder switching or guarding full time 2-4 in the majority of matchups.
Covington has looked good, at times, on a team with no intent on winning. No clue how he'd perform on a decent team. Crowder has been the fulcrum on a good team for a couple years now.
Noel can't possibly become a defensive player along the lines of Deandre Jordan unless he can stay healthy and become a much better rebounder. A huge portion of DJ's value on defense is his ability to dominate the defensive boards.
On offense, Noel is not a particularly good finisher, nor is he a great offensive rebounder. There again he absolutely pales in comparison to a big like Deandre or Whiteside.
Saric is nice, but it's hard for me to judge him by numbers alone given the team he's on. Players with his skill profile have to be pretty excellent to justify handling the ball so much. Players that fall short of excellent tend to be extremely situational bench contributors, a la old friend Evan Turner or somebody like Kyle Anderson in San Antonio.
I see the upside with Saric, but I also see a downside that you don't seem to consider.
As for Crowder, I think you're underselling the value of a genuine two-way, starter caliber role player at the wing position locked up long term to annual salary that is probably a quarter of what the player would get on the open market.
Noel and Covington will soon be each making at least twice what Crowder makes, and will make for another few years.
That's a huge deal when you're trying to build a good team.