His skillset doesn't include the most important skill.
Some bad team with cap room will overpay him.
Doing things that help your team win games is the most important skill set. He is going to get paid.
Oh, I've no doubt he'll get paid well. I don't think it will be by the Celtics. Some teams - mostly bad teams - will have a ton of cap space, and few players to burn it on. Someone will pay MS a lot more than he's worth.
And no, while the nitty gritty role Marcus plays is important, no skill is as important as being able to get your own shot, and/or be able to shoot 3s. Marcus can't do either. He's shooting 33%. Were it someone else, we could chalk it up to a bad start to the season. With Marcus, it's who he is. It's difficult to play winning basketball with that great of a handicap.
With Kryie, Tatum, Brown, Rozier, Hayward in the fold, I don't see the Celts getting into luxury tax over Smart. Fans can make some good points as to why we should, but I'd wager Marcus will be diving after loose balls somewhere else next season.