I think that Ainge's original plan was to add a lot of depth and make a respectable playoff run, maybe getting to the 2nd round. The team would look like it was progressing and building momentum which could be used as cachet for free agents or a potential deal.
The plan isn't working so well so far, for what could be a few reasons:
1) The East is more competitive this year, especially in the middle.
2) Our 2nd half run could have been a little fluky.
3) Players who were expected to improve have stayed the same or regressed (Zeller, Sullinger, Olynyk). Though, you could argue that Crowder, Thomas, and Bradley have improved.
4) Free agents/trade acquisitions like Lee and Johnson haven't panned out as well as hoped.
Whatever the reason is, I think if it continues much longer it really couldn't hurt to start playing some of the kids. Lee will probably be bought out and Sullinger doesn't look like he's part of the future. They're not exactly improving their trade value by playing, so sitting them wouldn't seem to do a lot of harm anyway.
The Suns are in turmoil now because they overachieved two seasons ago. Maybe the Celtics should take a step back this year and then re-examine their options this offseason.