Look at the Bucks too. When Brooklyn walls off Giannis' path to the rim, no one can break the defense. Jrue I am especially upset with. He can't get by anyone, and they just stick one guy on him. His offensive issues unfortunately perfectly complement Middleton and Giannis' offensive limitations making it so their offense doesn't work.
It's really what I am worried about with Garland. His outside shot isn't breaking the defense yet, while Sexton sees the 4th most double teams in the league. You can wish Sexton was the 6th man all you want, but no one else on the team is doing that.
Okoro is the only other guy I can see developing that kind of attention right now. His first step and finishing means it is possible. Garland needs to really kill with his off the dribble 3 for him to be that player.
He's a good passer and is developing as a floor general, but he can't break the defense with his passing, first step, or shot yet. I am not saying he can't develop that, but it isn't there today. That is Sexton's best skill. It isn't actually his scoring. He can upend the defense. It's also the reason people get upset with him, because he could take more advantage of it and he hasn't fully exploited it yet.
edit: Weepin Willow here is making my same point about Collin's decisionmaking/playmaking. Doesn't matter to me yet because he is making better decisions, and no one else has show they can bend the defense like that. Until they do we need Collin