Trading Brown and Smart for Beal actually makes the team much worse. Heck, just based on how important Jaylen is defensively to the Celtics, I think it can be credibly argued that a one for one swap of Beal for Brown makes the Celtics a worse team.
Much like Harden, if you are trading for Beal, it has to be to add him to the Brown/Tatum/Smart core not to replace one or two players in that core, otherwise, I am not sure trading for Beal or Harden moves the needle to a title all that much.