I love Smart and would hate to give him up for Millsap. However, the only way I could see Ainge doing this if he's taking a longview approach. I know it sounds weird since Millsap is turning 32, but I'm referring to the draft and it's high level guards. For example, we could draft Fultz, Monk, or Smith (which is the type of player Ainge would covet) and fill Smart's minutes accordingly.
So the starters next year would be:
Millsap
Crowder
Horford
Thomas
Bradley
With a bench rotation of:
Brown (who most likely would also be seeing some 4 minutes as well with his expected maturity)
Olynyk (assuming the price is right)
Rozier (assuming he makes another jump next year)
Fultz, Monk, or Smith (all seem like a good fit despite the different skills they possess)
Big man (physical veteran with the MLE)