Why will the Warriors do it:
The warriors need rim protection desperately. Williams is easily a top-10 rim protector in the league when healthy, a great rebounder and outstanding finisher.
They also need perimeter protection and Matisse is a true savant in that regard.
A matter of value: if healthy each one of those players is at least as valuable as Kuminga and probably more well being on a great contract. But although they are young they are injury prone and therefore risky. That is why the warriors get them in a bargain for a player that, while having a great upside, doesn't want to be there.
Curry has only two to three really good years in him. They should build around him. give him defense and depth.
Why will the Blazer do it:
These players while could be great doesn't fit the timeline. They have at least seven players that needs to get 30 minutes a night not including Grant or Rupert and reep.
Kuminga does fit the timeline. If Sharpe or Scoot doesn't prove themselves to be the players the Blazer wants them to be he can step up and be a star.
The Blazer fit Kuminga: outside of Avdija nobody can really create for themselves efficiently and consistently. Kuminga could and would enjoy the free usage of the team has to offer.