I think you hit on most of the reasons.
I'd be interested to know how important #3 is. I think that stars attract other stars. Does hoarding assets while passing on PG13, Butler, DMC, etc. play a role for players or their agents? Teams like the Rockets are always aggressively pursuing established talent. The Celts, not so much. Does that factor in?
How do we know this is true?
Danny aggressively pursued the top 2 free agents last summer, and landed one and by many accounts came close on the other.
Then, reliable accounts say we offered what ended up as the #1 pick for Paul George at the deadline. Same for Butler. We beat OKC's best offer for George and IND rejected us. Those things are hardly our fault. And what offers did Houston make for those same players? Were they objectively better than ours?
As I look at the Rockets roster before the CP trade, they had one player better than our star (IT), and a bunch of other guys who clearly aren't draws by themselves (unless you want to argue that Al Horford is an even bigger star).
So they pursued and got one guy in the last four years. We've pursued many and gotten one very good player, while coming close but getting turned down with some amazing offers to other teams. (Those are offers the other teams almost certainly regret not taking, and which Ainge very well could regret offering - never mind the Winslow deal). We made a very smart trade for a guy who blossomed into a major star, and other moves for players who've become top-level role players. This says just as much about Danny's team-building skills, as finding good supporting casts is as important as anything else once you have a few stars.
This is a small number subset of small numbers, but there's plenty of anecdotal evidence we've been just as aggressive as Houston. And we've certainly been in the thick of nearly every single media discussion and rumor mill for big-time players leading up to their moves.
I hardly see how a free agent could look as the Celtics and not think they are willing to make moves to get better. And for all we know, by this time next year Jayson Tatum could be a budding star that's a draw on his own.