If you don't have a single top 30 player than you're going to lose in the NBA. Until Boston got Isaiah Thomas they were a lotto team. Lopez might be top 40. Young isn't even top 50.
They were pretty heavily reliant on Joe / Brook / Thad last year, too, and made the playoffs.
As for the thing about top 30 players, I'm not so sure.
The Nuggets won 33 games this year despite not having a top 30 player. I'm not sure the Magic or Bucks have a top 30 player and they won in the low 30s as well.
I think you're right that a winning record in the NBA requires some significant talent, and / or decent coaching. But it doesn't take that much to be mediocre.
They had more talent in the rotation. And they were still an 8 seed in a much worse conference that saw the heat and pacers have tons of injuries.
Deron
Jack
Plumlee
Alan Anderson
We're all big contributors in their rotation and none of them were replaced.
Well they kept Jack, but I agree with your premise. Plumlee is a really good big off the bench and has strengths that can be very useful compared to Brook. He is averaging 10.5 points, 7,5 rebounds and a little over a block a game in 25 minutes on a bargain basement contract. While I realize Teletovic did not play a lot last year, he is a really good shooter and has been a nice consistent presence for the Suns. They really could have used him this year. Deron Williams has certainly been servicable for the Mavericks and represents a big upgrade over sloan or Larkin.
Adding insult to injuries, Barg, Robinson and Ellington really have not been as good as they maybe would have hoped after signing them this offseason to be in the rotation (Though Robinson has had a few big statistical games since Lopez and Young were benched). All in all their offseason last year was a total disaster.