I would much rather have Manu than Boozer. I love Carlos' game, but no way he puts up 20 and 10 next to Duncan. The only big in the league who I think would be a lock for that while playing with Duncan is KG.
Robinson was 10x the player Boozer was in 99 and couldn't do it. As for the rebounding, his numbers would definitely suffer next to Duncan. David was one of the great rebounders I've ever seen who never put up huge numbers (after 92) because he played the rest of his career with 2 of the greatest rebounders to ever live (Rodman and Duncan).
Make that trade and the passing on this team is nowhere near what it is now, the perimeter defense is worse, the fastbreak goes downhill, etc while the middle gets clogged 2002 style.
The Spurs made the right move.