Izayer won't be expensive, to be sure, but Ainge is not usually one to seek out a crowd-pleasing signing. I hope he doesn't start now, because Stevens' defensive schemes will work much better without going 4-against-5.
I am not worried about finding Celtics to shoot. Tatum, Hayward, Brown and (probably) Rozier can certainly chuck it, and would probably love the opportunity. If the defense is effective, the offense will follow (at least in the regular season).
I've heard the arguments for a strengthening Eastern Conference, but I'm not convinced that it is an irreversible trend. Philadelphia may lose Butler (and will probably sign Harris, which may be worse) and Toronto will probably lose Leonard. Boston doesn't have to be better than Milwaukee or Philadelphia to make the playoffs -- they just have to beat out Miami, Orlando, Detroit and perhaps Brooklyn or Indiana. In the East it is harder to miss the playoffs than to make them, and that bar hasn't gotten any higher.