Couldn't disagree more with the OP. Tatum and Brown can definitely play next to each other. Tatum is at his best when playing at PF. Traditional PFs aren't mobile enough to stay in front of him on the perimeter. This is a massive advantage in favour of Tatum, especially since he can put the ball on the floor and create his own shot. Brown is at his best when playing at SG. He's tall/long enough to shoot over most opposing SGs or take them to the basket. At the same time, he's mobile enough to stay in front of SGs on defense. Btw, the Celtics have made 2 trips to the ECF in the 4 years Tatum and Brown are playing together. I could even argue we are 2 out of 3, cause Brown missed the playoffs last season.
Today's NBA is all about defensive versatility (and shooting). If anything, we need one more starting-caliber swing. I mean, we never replaced Hayward. We also desperately need better ball movement/more passing. Long story short, we could have a use for a Point Forward. Easier said than done. Three names come to mind: Simmons > Siakam > Kyle Anderson. Obviously, they are different players compared to Hayward. Imo, Simmons is better suited at Point Center. Siakam ain't exaclty a Point Forward, but he's a willing passer. SloMo is the epitome of a Point Forward. I'd also be happy with Jerami Grant even though he's nothing special as a passer. If we trade for Jerami, I reckon we'd desperately need a pass-first PG.