My concern is so low because he's already done this transition from meh starter to 6th man and because there's only 32 games left. If we're doing the Conley deals we're talking about, I'm not even sure I have Sexton as a starter anyways. We could get no player back from Conley and you still don't have to start Sexton if you're so worried about his head getting too big.
For me, starting Sexton is like 99% positive and 1% concern. I want to give him that chance. I don't think we should really limit any young players opportunity out of concern that the opportunity overly inflates their ego. You could just as easily say he becomes unhappy without the starting opportunity as a 6th man who believes he's more than that. If Sexton turns out to be that dude whose ego is very overinflated, chances are he would have felt that way whether or not he's the starter for the rest of the season.