While Kyrie is not a Top 5 MVP contender, he is certainly a Top 25 player in the NBA. His loss will be felt. But,....
We know Kyrie can be the 2nd dog in an NBA championship team. He and AD would likely have been a nice fit and would have made the Celtics true title contenders. However, if you have to build a team around Kyrie being your top dog, it's not all that easy. He has a tendency to try to do it all on his own, and he's not in the league of players that can do that on a daily basis. Milwaukee was able to defend him, and we've periodically seen other players give him fits as well (IIRC, Avery Bradley did a defensive number on him one game). So far, the evidence of coexistence of Kyrie and another superstar has a sample size of one, and by the end they did not gel all that well.
For the most part, I've defended Kyrie's talk as being the typical banter of an immature, self-absorbed athlete, which is not a characteristic that Kyrie monopolizes by any means. Recall Paul Pierce's "I'm a great player on a bad team"; Ray Allen had a bit of a reputation as well in his early years. However, his explanation of his call with LeBron really bothered me, and probably really bothered a lot of his teammates as well. He could have just said "I reached out to LeBron, as I felt it was time to patch things up. We accomplished great things together, and he gave me some great advice for me to take along going forward." Instead, he goes out and publicly empathizes with LeBron's plight about playing with younger players. It was out of line for a guy that claimed he wanted to be a team leader. LeBron may be able to get away with it; Kyrie cannot.
So, while I do believe in "addition by subtraction" (it does work in some cases, even in the NBA), I certainly don't consider Kyrie's departure to be that. It will hurt the Celtics. And there was literally nothing that could have been done. Kyrie seemed happy here his first season, claimed he was here for the long term back in September, and then goes off the reservation for whatever reason. And it's not like Danny could have traded him at the deadline; it would have appeared like punting on the season, and I'm not even sure he could have convinced Wyc & Co even if he wanted to trade him.
But, the Celtics still do have assets to build around. It's a less optimal path, but circumstances mostly out of Danny's control forced his hand here. Sacramento being unexpectedly good this season hurt, and there is no way Ainge could have predicted that when he made the trade with the Sixers. Memphis has a chance to be pretty bad over the next couple of seasons, although the flattened lottery odds make any prediction of where that pick will end up completely worthless. I'm wondering if it makes sense for Ainge to swing for the fences on his first 2 draft picks, and pick 2 high-ceiling/low-floor players that most people are afraid of, and hope for the best.