2018 - 55-27, 17-11
2019 - 49-33, 1-1
2020 - 48-24, 9-3
125-69 (.644) without; 27-18 (.600) without
That difference doesn’t sound like a lot, but the difference in win percentages is about four wins per season.
But that is without replacing him at all and just removing him from the lineup. This is the exact same argument I was making with Kyrie his last year in Boston. That the team isn't better without him, but they aren't appreciably worse either and that is just removing him without replacing him with assets acquired in a trade. And low and behold, Kyrie leaves and the team is just flat out better by replacing him with a different, though lesser, player that fits better.
Defensive role players that know their role and don't over extend offensively are nice to have, but they don't move the needle much as they are role player. But that isn't even Smart. He over extends and makes terrible offensive decisions time and time again, which negates a lot of his defensive value. And because offense is so much more important than defense, it is a real problem.
Boston would be far more likely to win a title by upgrading from Smart to the #2 pick in the draft. Not next season, but probably stating in year 2 on, and since Boston's title window hinges on Tatum, it is far more critical to find the pieces that make sense around Tatum, and with Brown already in tow, Smart isn't that guy.