"They aren't as good as we all thought they could be" is not the same thing as "They're just not very good and it's about time everybody started looking at the team more realistically."
Irving is a top 15 player, I agree, but I would say he's a top 10 offensive player. He's among the most efficient scorers from any part of the floor in the league. He is more difficult to guard, in many respects, than any player in the East not named Kawhi. In some ways he's harder to deal with than Kawhi because he's a much better passer than Kawhi is. That changes if Giannis develops the ability to shoot pull-up threes, but thankfully I don't think he's gotten that down yet (lord help us if he does).
Horford is a top 20 two way player. His counting stats don't capture the full impact he has. He is great, even if he's showing his age a bit more this year.
Smart is a NBA All-Defensive 1st team type defender. He's Tony Allen with a jumpshot.
Jayson Tatum showed enough last year to believe he could be better than he's been so far this year. He hasn't really added very much to his game. That's been a disappointment. As is, he's in the Otto Porter tier of small forwards. Solid, efficient, not a game breaker.
Brown was steaming pile of dog crap at the start of the year. He's been quite a bit better lately. He's still deliberate with the ball and can't shoot free throws as well as he should, but he's figured out where he wants to get his shots (mostly in the post and at the elbow) and his spot up three has improved. He needs to make a more consistent impact on the boards and on defense.
Terry is flat out awful. He's never been very good (except for the playoffs last year, when he was a solid starter with some fun big game moments), but this year he's dragged the team down when he's on the floor.
Morris started hot but he's regressed, as you point out. I don't put that on Morris so much, since anybody could have seen that one coming. I do put it on the coaching staff that they didn't anticipate Morris turning back into what he's always been, which is a bench scorer whose confidence and swagger didn't quite match his ability to get buckets.
Hayward, I agree, is still a ways off from who he was. At least he's had games where he looks mostly like his old self, which provides some reason for optimism that he'll be able to play that way more often than not, eventually. But maybe not this year. I would agree that perhaps it was unreasonable to expect that he'd be anywhere close to his old self at any point this season.
Even with all of that, the Celtics began this year as the #1 defense in the league by a good margin. They regressed a bit from that but improved a lot on offense. For a long stretch they were a top 5 defense and a top 10 offense.
Yes, they lack an interior presence. That's why Baynes, God love him, is so crucial to this team despite being just a solid backup center and nothing more. But they've got a ton of switchable, versatile defenders and most of the roster can at least theoretically shoot a three. They've got one of the most difficult to guard 1-on-1 scorers in the league and a veteran star big man whose combination of strength, discipline, focus, and skill makes him a matchup nightmare for most teams, including the Bucks and Sixers.
All of that should get you more consistent winning than the Celtics have managed this year. It's the reason that they have, still, a pretty good point differential and many of their statistics suggest they're better than a team that's sitting in the 5th spot in the standings.
The problem is they haven't been able to put it together and go on any sort of run.
Yes, the talent on the team is flawed and hasn't lived up to expectations this year. But that's the point. They could have been and perhaps should have been much better. I think injuries have taken their toll, the coaching staff hasn't done a good enough job integrating everybody, and I think a lot of players on the team have allowed their impending free agency to stand in the way of their focus on what is supposed to matter.