I think that Ainge looked at the current roster and thought that adding a PG or JB would still make the Celts a finals loser at best. At every position, the C's would be a "poor man's Golden State." Curry>IT, Thompson>Bradley/Smart, Durant>PG or Butler, Green>Crowder, with center a wash at best. And that doesn't take into account the real drop off with players traded to get PG or JB.
I wonder how much the uncertainty of the exact place of the '17 pick hurts its value - it has to be pretty big. I'd imagine it's a lot easier for a fan base to swallow losing a star player if they know they're getting back the #1 pick in the draft. If Ainge is fine with whomever is available at the #4 pick, then trading that pick now would be selling low.