According to nick, the Celts could feel that adding a guy like WCS, Noel, or Poeltl won't make a difference against the likes of Embiid and Giannis.
Here's the thing, although Kanter is better than all those 3 overall, those 3 Centers are also all better than Kanter on the defensive end.
If Kanter is only decent on defense, the Celtics' problem of needing a big man is solved.
Unfortunately Kanter is worse than Olynyk on defense.
That's why the Celts need someone like WCS, Noel, or Poeltl.
All 3 are better than Poirier and all 3 will do better on the defensive end than Kanter.
So why not upgrade if it will only cost the Celtics Poirier and a draft pick?