With the dust settling on this trade a little bit, I am starting to feel less depressed about it. As a few have already pointed out, Brooklyn might really struggle on the defensive end. So even if you get the personalities to gel, Kyrie to actually want to play, and the team to really buy in on Nash, they are far from a certainty. We will just have to wait and see.
The Celtics in some perverse way are in better shape now than they were at the start of the season. Why? Because Milwaukee and Brooklyn have gutted their future draft capital and shortened their potential windows. For the Bucks, of course they just extended Giannis, but Holiday is already 30 and is a FA after 2021-22. For the Nets, Durant looks like a monster right now but is 32 with a reconstructed achilles and, similarly, can hit free agency after next year.
The Celtics aren't the favorites the Bucks and Nets are at this moment to win this year, but they *could* win a title this year if things shake out right. Meanwhile, two years from now you'll have Tatum and Brown in the middle of their current contracts at ages 26 and 24, respectively. By then, either they'll have added a 3rd stud to pair with those two, or they'll still have draft capital and cap flexibility.
It's hard to think this way because we generally want satisfaction now (which, again, I think you still may get if you're a Celtics fan), but the period that comes next in the East looks especially bright, particularly if the team can sign Tatum and Brown to stay on beyond their current contracts.