Boston is sort of in the middle of a championship run. A bit unbelievable given how just okay they are. God, this era sucks for good teams. Anyway, one thing that's hard for teams in their position is retaining good role players. You run out of cap space and flexibility. Horford won't last forever and their other pieces are just fine.
Marcus Smart is basically the heart of the team and he's 28 years old. Without him, Brown and Tatum seem to be playing in two different worlds. Again, how this team is a contender is beyond me. They barely have anyone of note beyond these guys.