I may have miscounted by one or two, but by this source I count 14 head coaches that were calling the plays going into 2024. Offensive coordinators called plays for 18 teams. It's not unusual. Whether it's a good idea or not is debatable. But some very good head coaches that are good at it seem to get their teams in the playoffs fairly regularly. For some teams the head coach is the real OC with a proxy up in the booth that they call the OC, which is basically what Schott is, and several others.
https://www.espn.com/nfl/story/_/id/41018846/nfl-playcallers-32-teams-mike-mcdaniel-sean-mcvay-nathaniel-hackett
So if we want to find any room for hope in all that, my stance has been that Mike not calling plays next year may not be a bad thing, and we will find out if Brian is better at not only calling the plays, but with his own version of the scheme. Fans often bitched bout McCarthy's play calling. For me it was mostly how bland it was with not enough wrinkles to get receivers more open, give the qb better checkdown options, and open things up for the running game as well. Mike's game management wasn't great, either. Some new blood on the staff might bring a few ideas that Brian may be more open to using.
We'll just have to see.