At the end of it, it's always the same thing. People underestimate team play and chemistry and keep talking about individual talent and judge a team by their individual talents and make the sum of them.
We get better by trading Rondo & Green. It happens they were our 2 best players but there level of talent wasn't the problem. It was their attitude towards the team or the coach that wasn't suitable for our current roster. A good example is the Kings, something is wrong since a few years and they can't figure out why, they keep shuffling their roster and they fail each year. I don't think Cousins is their problem, but more their ability to find a good coach that Cousins will respect and a good GM that knows how to build a team around a special player like Cousins.
Going back to our roster and their performance, they play their best basketball because they probably trust the coach and each others. This is 50% of the win. To get an edge and have a better team you need talents. But you need that team play as a solid foundation or else you are nothing (look at the Cavs first part of the season, until they finally find a way to play together, they were very average).
As for next season, don't underestimate Amir & Lee and what they can bring to the Celtics. Media didn't pay attention to Amir and he was buried behind DeRozan & Lou. We are very lucky to have him, he's a perfect fit.
We'll be a match up nightmare for most of nba teams. We won't win any games with big show and athletic performance nor ridiculous 3pts shooting %, we'll win games collectively with hustle and top intensity level because we have a roster that is built that way.
You can say they won't win 40 games, that's your opinion, but don't say they won't be good because they don't have talent. To me this is a proof of not understanding our team.