I disagree. I think Millsap was ready to blossom but we held him back. We were playing him at SF so Favors and Kanter could get more minutes. He was the odd man out in terms of how we didn't feature him, and the guy still had a season where he averaged 17 points, almost 9 rebounds and 2.5 assists.
https://www.basketball-reference.com/players/m/millspa01.html
I definitely agree that Millsap would not have made it to 4 straight all star games in the west, but I do think it was a massive mistake letting him go. The guy has been a really good player and a better player than Kanter and Favors. And it's not even an argument - he would have been a much better fit next to Gobert.
We could have not only kept Millsap, but we could have potentially traded Favors AND Kanter for a lot of value back in those days. We drafted Gobert then just let Millsap walk a couple weeks later. If we would have re-signed Millsap and traded Favors and Kanter, we could have had a very, very good team. Hayward, Millsap, Gobert, whoever we got in Favors/Kanter trades.