The NBA is messed up with this CBA. It allows teams to give MAX contracts that forces a team with 2 good players to use 70% of their salary cap for those two players. Once you're there, it's almost a guarantee that you're going to have to pay a hefty luxury tax just to put 5 players on the floor.
If you're a team like the C's, with all 5 starters making big bucks (starting next year), you're screwed by this salary cap. With Brown & Tatum making over $53 Million each, there is a little more than $80 Million before they're over the luxury tax threshold and only a little under $89 Million left until they're on the first apron, which screws them when they try to make moves to reduce their cap $$, which seems counterintuitive to me. Hey, you're over the cap, so we're going to prevent you from unloading salary to get back under it.
Getting out from under KP's $30.7 Million is not going to be anywhere near enough to lower the C's luxury tax or even enough to get them down off of the second apron, which severely restricts what they can do on the trade market. They would need to shed over $54 Million to get off the second apron.