Smart leaving the team is as pivotal as Holiday joining the team. Smart was a great and fierce defender, true. But his decision-making at the end of ball games played a large part in many Celtics 4th quarter collapses over the years and in the playoffs. Meanwhile Holiday's experience winning it all and bringing that professional presence to the locker room surely helped increase our Championship odds.
Bottom line: This Celtics team could NOT finally become the Jayson and Jaylen led team until Smart left. And that's not denigrating Smart, that's just stating the culture needed to change and the mantle passed on to those two for this team to find its true championship identity.
Your argument already sounds logically weak when it starts with, well Smart being here would not be SO problematic as to PREVENT us winning a title. Seems to be a bit of grasping at straws.
It is true that Smart has a little of that Draymond Green or something in him, where he drives his teammates crazy sometimes, but they know that he will fight to the end for them. Brown says he loves Marcus. Tatum said he is a different guy but you learn to love him.
I am not convinced that Marcus Smart would be a locker room problem to the extent that it would prevent a team from winning a title. As you said, it is just our opinions.