The team is different with Smart, but not necessarily worse. I think the offense is worse, but the defense is better. This year, in particular, the team is significantly better when Smart is on the floor then when he is on the bench as his on/off per 100 possessions is +6.9, which is 2nd on the team to only Tatum (+9.1). That is a little better than years past, but in general Boston is better with Smart than they are without him. That said, I think you could replace him with a lesser defender but a better offensive player and the team could get better. Schroder just isn't good enough offensively or defensively to really alter that narrative, but I do like the offense better with Schroder out there as he is an actual PG, something Smart is not.
With the team as currently constructed, I think they need to go with Schroder, Smart, Brown, Tatum, and Williams (I'd say Horford but he isn't shooting well enough to justify him starting). I don't really like that lineup, but that is the best one for the team in its current state.