But that would make sense only if Pop had been playing Rasho a lot to begin with. He hadn't.
Your comparison with Duncan would make sense if Rasho had ever been an elite rebounder, shot blocker, or offensive player. Rasho was never any of those things. Tim Duncan is among the 10 greatest players in the history of basketball because he's a dominant rebounder, a dominant shot blocker, and a dominant offensive player, whatever his athleticism might be.
It's about results, and if Rasho could have shown results no matter the disparity in athleticism, I'm sure he would have played. But he didn't show results -- except to a few Spurs fans who somehow have sainted Rasho with rosy fantasies of defensive dominance. Rasho was a fine defensive player when matched against bigs who were basically the same as him athletically; when matched against players who were much more athletic, Rasho wasn't particularly productive.
Oh trust me, I think the Spurs' FO makes mistakes. By coincidence, while the Spurs have won more games than any other franchise in sports over the last 15 years or so, though, I find that their mistakes are basically limited. Teams that have the sort of successes that the Spurs have enjoyed don't make a lot of mistakes.
I just think the things that you complain about are really poor arguments.
I'll admit to this, though: having been a Spurs fan when things were really bad -- when the front office regularly made hideous trades, when the team wasn't a lock to be in the playoffs, when playoff runs were ended summarily by much, much better teams, when every decision that the Spurs made seemed to go to crap -- I can say I'm fairly content with what Pop and his crew have done.