I think some also forget that Tatum only has 2 years left on his deal before he can opt out. He's not guaranteed to resign when the time comes and we shouldn't just assume he will be.
And Scoot might be good. People also predicted that Markelle Fultz was going to be the next great point guard.
I believe that is correct that Tatum has only 2 years left (2023-24 and 2024-25) but then a player option in 2025-26. He is eligible for a supermax extension so long as he stays with the Celtics (I am not exactly sure how the option year plays into that). Nothing is guaranteed of course but I think it is pretty reasonable to assume it is highly likely that Tatum stays a Celtic, at least for the next contract, a supermax contract.
And the idea of trading Brown for a draft pick, such as the #3, I think the 2017 draft is a perfect cautionary tale. The draft went:
Fultz
Ball
Tatum
Jackson (Josh)
Fox
So in the top 5, there is only one that you can say for sure turned out better than Brown (Tatum). Then you have Fox who is good, comparable to Brown, but not quite as good. And then 3 that are clearly disappointments, would result in the trade being a terrible trade. Those are about the odds every year. In the top 3, there will be a star (maybe, and you still need to wait a few seasons), a decent player, and a dud. Every year is different of course, but this is about the norm.
Why would the Celtics want to trade an already top player for a role of the dice to maybe get a player who at best will not be as good as Brown for a few season and more likely will never be as good as Brown? It just doesn't make sense to me.