And Duncan never had players as good as Pierce and Ray. (Robinson was past his prime when they played together. Still good defensively, but lost a step offensively. Much like KG this year)
Robinson was better for the early part of Duncan's career than Pierce or Ray have ever been. That's no disrespect to either Pierce or Ray, Robinson was still that good. And Ginobili/Parker have performed at least as well in the last couple of Spurs postseason runs as Pierce/Allen did in '08.
Early on
Googs and Marbury
Then Brandon, Smith and Wally.
Then they added Billups
And finally, the Spree Cassell team.
None of those guys are the on Pierce's level in terms of taking over at the end of the game, and that hurt KG's playoff winning chances.
Duncan never needed that other player to do that.
It should be telling that you were listing the absolute best that KG had to work with in 12 years in Minnesota, and even skipping the really bad years you still had to list rookies (Marbury was 19 the only year that he played with KG and Googs), inexperienced at-that-time journeymen (Chauncey was considered a bust and on his last legs, signed to a near minimum deal to play back-up before he resurrected his career next to KG) and role players (Joe Smith? Really?) among the short list of "best and brightest". As someone else pointed out, Duncan's teammates were consistently more talented than KG's. Usually by a lot.
And Ginobili (and to a lesser extent Parker) played the "Pierce role" on the last couple of Spurs championships. And Duncan absolutely needed them in that role.