I think reading between the lines from Brad Stevens and Tatum's comments on the offseason moves, a major factor in moving Smart was to give Tatum and Brown full control of the team. They deferred to Smart too much, especially late in games. Smart, as others have said, has been declining too. His defense was mediocre last season. White is a much more natural fit at PG, and adding Porzingis is huge for this team as Horford can't be a full-time starter on a contending team anymore. With these changes and the improvement of the coaching staff, I think we improved a great deal.
I also would not underestimate Jordan Walsh. I know he's a rookie and will have a learning curve, but he looks like an excellent fit for this team at the wing spots.