Who says no, and why?

I can easily see this happening. It works for all parties.

Golden State gets off Kuminga and gambles on Porzingis. While replacing Harris for Hield and Melton. And getting a SRP.

Milwaukee gets a haul for Giannis and reboots.

Atlanta makes a push. This makes them top 4 in the East.

Detroit helps facilitate and gets a rotation shooter in GTJ, to help space the floor. And he fits the youth timeline.