Its a great sign that Smart has focused on fitness and losing weight. He also seems to know he needs to simplify his shooting mechanics.
This is a different kind of year for Marcus. Until now, he was more or less cemented into a third guard role and got his 30 minutes. There was no one on the roster to take his minutes.
This year, he will be competing with a rapidly improving Jaylen Brown as well as Rozier, who has the tools to be a much better offensive player than Marcus, but just hasn't performed consistently yet.
Hayward will probably fill some minutes at shooting guard too.
Whether he is worth $10M or $20M depends on his competition with Brown and Rozier. If he has another year where he is a dismal shooter, he may very well find himself behind both these guys and he will be defined as a nice role player who lacks an NBA offense.
Rozier, and especially Brown are pretty likely to make major leaps forward in their games. Marcus can't just be a little better, and expect to keep all his minutes. However, if he meets the challenge, and outplays Brown and Rozier, he'll be in for a big payday, and ironically, we probably won't be able to keep him.