This is actually a pretty fair trade if you think Simmons is the real deal. Simmons would fit in well as a playmaking 4 on this team, and it would save us from the uncomfortable logjam of guards. However, I think the risk on Simmons is greater than Fultz due to him missing all of last year and the fact that he can't shoot for crap.
Is it very possible the player taken this year at 3 (Jackson/Tatum?) will be better than Brown, but even so Fultz is too valuable to give up. You'd have to really think Jackson or Tatum was a future star to convince me this deal is worth it.