If you didn't see this coming you haven't been paying attention to the NBA in recent years. Players first sign for the absolute max, and then if they so desire they demand a trade. Teams that try to resist giving into the players' trade demands always end up caving in. The 76ers lasted the longest with Ben Simmons this past season, but of course ended up giving in.
$50 million a year is way too much money to be paying Bradley Beal for his thirties and if the Wizards are smart they will trade him and rebuild properly as soon as they can. The Wizards would have been wiser to trade Beal last season.
In terms of the Celtics, I always knew they had no shot at Beal unless they had the ability to clear max cap space. The threat of Beal signing in Boston, so the Wizards should trade him now for something rather than lose him for nothing only worked if Boston had the cap space.
Every move Stevens has made since re-signing Marcus Smart and Robert Williams to extensions to trading for long term contracts in Derrick White and Daniel Theis has shown that the Celtics did not plan to have cap space.