All the regression to the mean talk is a bit overstated. Yes, there are certain factors that may come back down to earth- after all, we're not going to with 80 in a row.
But, a couple of things:
1) It's very hard to define what the "mean" is with a group of players that have never played together. Irving, Tatum, Morris, Baynes, Semi, Theis, Yabusele all brand new. Brown is a significantly expanded role. Horford being used differently. Yes, if opponents appear to be missing more shots that they should that matters, but it can also be the new normal when practically everything about the team is new.
2) All the focus seems to be on our overachieving defense, and then how our offense isn't that good. Our defense might get worse, but our offense is just as likely to get better. Do we really think Smart is going to shoot 27% all year? Irving is going to shoot a career low from 3PT? Morris has only played 7 games. Rookies won't improve? Rozier is going to shoot 35%?