Yeah, other than cash, the thing they stand to lose by going and staying over the 2nd apron is flexibility. But with Tatum, Brown and KP, they shouldn't need to add a star, and maybe they think if they can line up the right core of role players around those guys for the next three years or so, then flexibility be damned.
Looking at the roster, they've got a really solid group on paper of guys in or about to enter their primes. A list of role players that includes Derrick White, Malcolm Brogdon, Robert Williams, Grant Williams, and Al Horford is pretty good surrounding those three stars. I still think they need one more rotation-level wing, but they're close.
I'm surprised that the prevailing sentiment seems to be that the long-term outlook doesn't include Jaylen Brown, whether they sign him to the Supermax or not. I look at it more like this is the offseason to construct the best possible role players around your stars, because after this, they lose the ability to add much talent barring a really creative trade. I expect them to sign Grant, add a two-way wing with the MLE, and go from there. I'm well aware that that's an expensive roster, but I haven't seen any indication that they're willing to undercut the talent on the floor to save money. Not yet.