Jaren Jackson Jr's contract is an overpay, sure, but he and Bagley are also very different players. Sure, over their careers JJJ has averaged 15.4 and 4.7 while Bagley has averaged 14.5 and 7.5 and both have missed a lot of games due to injury.
But the differences are that (1) Jackson Jr plays a modern NBA big man game (2) Jackson Jr plays defense (3) Jackson Jr is a good fit with his teams best player.
One of Bagley's only areas of improvement is his outside shot. Last season he averaged 34% from outside but it was only on 2.5 attempts per game. His previous two seasons he averaged less than 2 3PA per game and shot a worse percentage. JJJ missed almost all of last season and had poor shooting numbers on a small sample size of games, but the season before he shot nearly 40% from 3 on 6.5 attempts per game.
Bagley still gets wide open looks from outside because he's not a knockdown shooter and teams would rather give him that shot than close hard and have him go inside where he's more effective. JJJ is a better floor spacer right now.
Jackson also is a better and more active defender who gets more steals and blocks than Bagley but more importantly, is much more aware on that end of the court. Bagley still looks to me like he gives token effort on defense but is really just waiting to run out on the break for easy baskets. It seems clear to me that that's where his focus is. He's definitely a much better rebounder (JJJ is pretty abysmal for a guy with his size and length) but otherwise, Jackson both spaces the floor better and is a plus on defense rather than a minus.
And both of those reasons are why Morant and Jackson Jr are a better pairing than Fox and Bagley. Morant needs a floor spacer to open up the lane, and Jackson's shooting and defensive versatility allow him to either play the 5 in small lineups or slide next to a full size center without creating spacing issues on offense and switching issues on defense.
Bagley SHOULD be a good pairing with Fox as a big that can really run the floor and finish, but his defensive shortcomings, inability to consistently space the floor, and tendency to be a black hole with the ball in his hands mean that three years in that duo hasn't worked well together.
I've said for the last couple seasons that Bagley is much more of a throwback PF (with poor defense) in a league that has largely phased out throwback PFs. Without great outside shooting his best position is probably at the 5 but you can't have him as your defensive anchor and expect to win games.
Bagley shouldn't start games next season because of how it might impact his QO. He also shouldn't come off the bench for the same reason. The only thing that should dictate whether Marvin starts or not is whether putting him in the starting lineup gives the team a better chance to win. Unless something changes dramatically this season, I don't think he does.