If Beal is a "much" better player than Bradley, why would Washington be so open to trading him?
Fit, timing and flexibility. My assumption is that the Wiz are going all-in for Durant this offseason. If Washington is all-in for Durant, they're going to tie up a lot of cap space. They're also paying Wall $17m next year. Beal is a RFA so Washington has the right to match but they could have issues with the timing of offers. That's not the biggest reason though...
A guy like Bradley gives them much more financial flexibility to add people in other areas. If the Wiz are paying large sums to Beal, Durant and Wall that leaves them little money to fill in on the front court. They'll be able to score but will have trouble in the playoffs because they simply won't have the bodies up front to compete. Look what happened to them in the playoffs last year against Atlanta. They got dominated up front. That probably would continue to happen by paying most of the cap to Wall, Durant and Beal.
Bradley's game may work much better with Durant and Wall. Both of those guys tend to like the ball in their hands. So does Beal. There aren't simply enough shots per game to keep all 3 happy. So instead you bring back a guy who doesn't need the ball in his hands, can hit the 3 and who can play great defense to free up Wall on the offensive end. Bradley's game is more mature as well and Durant will be in win-now mode.
Now add on top of that a 1st round pick or two and the Wiz would be able to continue to keep the pipeline of young talent coming and/or make another move to improve their front court. In that way they don't necessarily have to sacrifice their future in order to win now.
It's really not much different than OKC trading off Harden. He simply was too costly and not a good organizational fit. Beal would be very similar if Washington thought they had a good shot at Durant.
Now don't get me wrong - I'm not saying this is the right move for the Wiz - but I could see this trade being beneficial to both sides.