I agree and Boston will regret that trade.
I think this playoff run has cemented Tatum's status in Boston. Beyond that, Kawhi is rumored to prefer LA, NY, and Phillie as his destination. Given that he's a FA after next year, another team is unlikely to trade for a rental, and SAS won't give him away for a handshake. They'll get real assets in return, or they'll force him to walk and leave millions on the table. As such, IDK if he'd be willing to go to Boston. If he were, I'm no longer sure that Boston would carve off one of Tatum or Brown (probably Tatum) to get the deal done.
The guys that need to be worried are Kyrie and Heyward. Brown, and Tatum look like the core of a future dynasty, and they are the type of versatile, capable of defending 3+ positions, good perimeter shooting wings that the NBA covets. Teams are gagging for at least 1, and having 2 under the age of 22 is manna from heaven. If I'm Boston, I'm looking down the road for, say, 2 years, at which point the current Houston team will have aged out, and Cleveland is likely to be imploded or diminished along with LeBron. GSW will be aging, and chemistry might be fraying. The Phillies will be good, but Boston will be right there. And the Phillies future rests entirely on the oft injured body of Embiid.
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Boston shouldn't go ALL IN on winning in the next 1-3 years. Towards the end of that, a near-decade long window of opportunity is likely to open.