This is one of those trades that, if it was a video game, I wouldn't hesitate. Beal is better than Brown and still only 27 which is fine.
But in reality, I love Jaylen Brown. I like that he's only ever played for Boston. I think he's good enough to be a #2 on a championship team and he keeps getting better. It would be more satisfying to keep Brown and win a championship with him than trade for Beal and then win one.
If I thought a championship was very unlikely with Brown, then I'd want to make the trade because the ring is the thing but, even though Beal is better, he doesn't raise the ceiling enough to offset my attachment to Jaylen.