I also have that same opinion about Celtics, they don't need KD, the only thing that they needed to do is to continue doing more progress with their system, they've made it to the finals last season the players deserves kudos for doing their roles and now they will again try to push for another shot in chasing the world title, having KD and breaking the good chemistry might not be a good idea for the Celtics' managements.
However, Boston Celtics show interest in Kevin Durant and even consider putting Jaylen Brown on the trade package.
It means the Celtics are willing to break their core just to get Durant even though Jaylen Brown was a big piece during their last Finals run. But since the Brooklyn Nets want more big fish aside from Jaylen Brown, that ends the story now, or...
I'm now even more concerned about Jaylen Brown. Even though NBA is business and players must accept that they might be shipped away even how valuable they are on the team, it gives Brown a disappointment that the team who thinks he values him is willing to just trade him away easily without even considering. I hope that won't affect his performance on the team which I doubt it really will.
TBH, I'm a bit surprised that the Celtics are interested in getting Durant to the point that they will trade their core players just to get him. They reached the NBA Finals without him so what's the point in getting him? They only lost to Warriors because of experience that's all but I believe this current core players of the Celtics will get a championship in the future. That's how promising these players are.
On the other hand, the Nets still want more players in the Celtics and that might lead to a point where this trade talks will end out with no trades at all.
Now with regards to NBA being a business and disappointment, I believe that the players knew about it at first place but as a human being, there will still be a bit of disappointment at first because of the bond that you've build in your former team but that's part of the business and they have nothing else to do but to accept. I still remember the time where in the middle of the game, Harrison Barnes that is on that time on the Mavericks has been traded to the Kings. You can see the disappointment in his face at that time but overtime, they will overcome it and will accept and that is the same with other players.