That doesn't make them "really good players". They might have been the best the team could get, in terms of caliber, but they clearly targeted centers; it didn't just happen by accident. Oberto was undersized, but his game was clearly that of a center's.
Again, you're acting like Aldridge can do what Duncan does, just because of a similar physique. It doesn't work that way. You can have all the mobility, quickness and switch ability you want; if you can't protect the rim/defensive rebound, then it's all for naught, as evidenced by the games Duncan missed.
No, I argued Young was a natural three and he is. The only difference between him and guys like Gallinari, J. Green, Gay, etc., is three-point shooting. Horford has been a center because fellow top 2-3 players on their team were power forwards, who couldn't play center. In Bosh's case, they couldn't even find a second legit starting big at either position, until a year ago. Now, he's back to playing more power forward. By your logic, Duncan is a power forward, because he mostly started next to true centers.
Everyone does better offensively when there's more space. That doesn't mean you start lineups that can't protect the rim/defensive rebound at the expense of it. You make it sound like Aldridge is Splitter, when it comes to shooting. There's roughly a half dozen players in the entire league capable of protecting the rim and spacing the floor and the Spurs aren't getting one of them, so excluding that pipe dream, they need a legit center next to Aldridge. They'll be games where said center barely plays, but the roster still needs to be built in such a way that they, like most teams still do, start that way and then adjust accordingly based on the match-up.