It's really hard to argue against this.
Before last season, we could say we were waiting on Demarcus Cousins, Jimmy Butler, Paul George, even Russell Westbrook.
Now, it's all down to Gordon Hayward. As you say, he's not even a top 10 player.
So I think you're right to question -- what is the plan here, exactly?
The answer at this point seems to be that the Celts want to have a team that will be an above average playoff team for a long time, and they aren't willing to give up their future assets for a short term boost that may not get them past the top team anyway.
After all, they were willing to chase Durant.
But give up a big cache of future assets for somebody who is less than an MVP type guy? That doesn't seem to be something Ainge is interested in doing.