As someone who pays real cash for tickets and goes regularly, the idea of a lengthy rebuild will likely make me give my tickets up. If I want to watch high level basketball, what I get on my TV is better than watching a 25 win team in person. The in game entertainment stuff just does not interest me.
At the same time I realize that this is how things get done in the NBA. The Celtics have 3 things in their favor - good draft picks, competent management and a new collective bargaining agreement that likely means fewer long term contracts so there's going to be more flexibility. Hopefully this translates into a shorter rebuild, but luck is also gonna be required and failure to me means becoming the Atlanta Hawks - mid level team with no hope of winning it all.
Doesn't make me thrilled but that's the NBA. I just won't pay money to see a weak team lose a lot more often than they win unless I see light at the end of the tunnel. Right now I just see a tunnel with a small spec of light in the distance.