This is the thing about drafting. You literally never know. Cunningham could be a future star or he could be the next Beasley, Fultz, etc. For example, coming out of college, I saw Beasley and Durant both as stars. To me, the way they moved on the court, their vision, the appearance that physically, they were men among boys, the smooth shot, all of it looked good to me for both and apparently did also to the majority of NBA talent evaluators. One became a hall of famer, one became a bust.
No matter how many times someone says, that they know Cunningham will be a NBA star, they simply don't. No one knows.
With Brown, you do know what you have as an NBA player. You don't know for sure how much better he may get but you know at worst, he is as fringe all star (fringe meaning towards the end of the list of all stars) who is still young and still improving. I do not trade that for a draft pick.
A player like Wiseman with only part of an NBA season has demonstrated what he can do in the NBA. He would be far less of a risk than someone like Cunningham. There was a thread discussing trading for Wiseman where it was Smart and RWilliams. I am not sure I would trade Brown even for Wiseman but if I ended up with the first pick, I would trade that for Wiseman. Cunningham MIGHT end up the better player but there is so much less risk or uncertainty as compared to Wiseman.