Garnett is argueably one of best 10 players to ever play the game so he has a point.
If you look at him as a PF he is right up there with Malone and Barkley after Duncan.
Garnett is at least 7'1, I don't care if you think he is not a true center or whatever. He can guard any big man in the league as good as anyone else except MAYBE MAYBE Shaq.
KG is a heck of a talent, but no way he is in the top 10 all time NBA. Maybe top 50, and there would be great debate about that.
He would either work out great with the Suns, and we would win multiple titles, or it would be a train wreck.
Assuming getting him were possible and required Marion, the ego question between KG and Amare would need to be clearly settled, and both would have to be comfortable with their roles.
Then, the team would have to be sure they could create an offense that works to Amare and KG and Nashty's strengths. Doable, but it better be thought out ahead of time. Then the question would come up of whether the rest of the team fits into this new offense, or if there would have to be a 3 year trading-out process to build the team up again?
I'm not sure I give up Marion and #4 for KG. The problem is that Marion for KG stat-wise is a swap. Sure, there are different things on the table with those two - defense on smalls vs defense on bigs, late game offense, one-on-one offense - but stat-wise it is a swap. That is a problem, because we need a better bench. Getting KG doesn't solve for that.
Keeping Marion and using the #4 pick very, very wisely? That solves for it.
Remember, this is a deep draft. We snagged talent like Marion with the #9 pick. If we get (cross fingers) a pick in the 4-6 range, and use it correctly, even as a rookie a player of that caliber ADDED to the current team would do wonders.