Honestly it's more like the season from "Meh."
The season is disappointing precisely because the pre-season expectation was that they would win 55-60+ games and be in a position to contend for the Finals, if not a title.
Instead, they've been a merely "good" team that quite obviously, at this point, lacks the "extra gear" or focus to be "great."
The Celtics are a good team that will make the playoffs and may enjoy home court advantage in round one. Hard to call any season that ends that way as a major disaster.
At the same time, they're clearly not a *serious* playoff team.
Serious playoff teams don't drop so many games to bad opponents on random nights in the regular season.
Serious playoff teams especially don't allow games against quality opponents to get away from them the way the Celts have numerous times this season, most recently at the end of the 3rd / start of the 4th against the Nuggets.
There will be major questions this off-season about what the core of this team will be moving forward, since the core group so deeply underachieved this season.
It has been quite a long time since a Celtics team fell 10+ wins shy of their pre-season Vegas over/under. Even longer since a disappointment of that magnitude that wasn't tied to tanking or injuries or both.
All of that said, the content in the OP is way over the top. There's plenty to feel good about with where the Celts stand moving forward. They have plenty of draft assets, plenty of talent on the roster, a decent cap situation, a very good (if not great) coach, a great GM, and one of the best ownership groups in sports.