Biz isn't that much of a downgrade, but yes, he is a downgrade. We'll never know how much a downgrade for the playoffs though, considering McGee was a net negative every time he touched the floor.
Even with Dario, rim protection will be our biggest issue, but can you remember just one game where McGee's presence actually won us the game? I'm sure there were one or two, but I definitely don't remember them.
Dallas has given McGee the chance of a starting role. There was no way we were going to be able to offer that, and we wouldn't have paid him 7m per either. The Suns knew of that potentiality and still didn't do anything to try to retain him. AND that is with us not being serious in Durant talks. Sure, the word at the time was that McGee didn't want to wait for the Suns to figure the Durant thing out, but the Suns were OBVIOUSLY going to match Ayton and it looks like there was never anything serious about Durant, so it's not as simple as the Suns lost him because we were too busy waiting on KD.
And at the time, we had NO centers other than Dario on the roster. If we thought he was that big of a difference maker, we would have come up with a way to keep him.