The way I see it, the Celts have one clear-cut All-Star in Isaiah Thomas. He's got at least 2-3 years left playing at that level, barring major injuries.
Aside from Thomas, the Celts have a number of nice, reasonably paid role players. Bradley, Crowder, Amir, Olynyk, Jerebko, Smart. Turner, Zeller, and Sullinger have been bargains but will get overpaid this summer in all likelihood.
Then there's a handful of rookie contract guys who have shown little to nothing so far. Might be rotation players someday, might not.
I don't think it makes sense to completely "blow up" this roster. At the same time, nobody on the roster should be off the table in a trade for a top 10-20 player (e.g. Butler, Cousins, etc).
Other than Isaiah, I think every spot in the rotation should be looked at this summer. The current group is nice but it's not good enough, and simply settling for the same basic thing plus a top 5 prospect shouldn't be acceptable.
If nothing else, shuffling some of the current supporting pieces around will be necessary to make room for the huge influx of rookie players (plus the ones that have rotted on the bench this year) if Ainge is unable to get some value in trades for all of these draft picks.