Good point, though I must say I'm not exactly excited at the prospect of that happening.
What you describe is pretty much the definition of mediocrity.
Luckily, the Celts aren't locked into this current group so the treadmill danger is not there, but why waste a season doing that if it's not building toward something?
I understand the fear that we might become the next Atlanta; it is a legitimate concern given that we have no LBJ, Kawhi or ADavis.
Basically, Ainge has found a bright young coach, a good core of players and is hoping that he'll eventually land an all star caliber player through FA/trade/the draft.
Things
can go wrong and three-four years from now we might have to confront our failure to build a contender.
But building a contender from scratch is super difficult
anyway and given we just finished year two of the rebuild we are doing pretty well: you can ask Detroit, Hornets and Toronto fans whose teams have stuck in mediocrity for much longer (I won't even mention poor Utah);
IMO a couple of years in mediocrity is a necessary risk in building a contender: there is no automatic switch from tanking to contender mode. Especially given how much depends in bball on getting a real star to build around (and given the NBA is a closed and highly regulated market with 30 teams all going after the same stars, there is no risk-free method to get your building blocks.)
Therefore, patience. After all there is intrinsic value in winning games even if it does not automatically bring you closer to being a contender: experience and self-confidence for young players, building a good reputation for your franchise, making the games more fun for all of us to watch.