Let's see: did the Blazers play pretty well when we had Blake, despite being riddled with injuries? Yes. So maybe just maybe it was a good thing to play him? And maybe just maybe the fact that Miller is playing well now isn't proof that it was wrong not to start him, but perhaps partly a result of easing him in slowly.
I remember Deron Williams's rookie year: he didn't start, and he was playing behind Keith McLeod (I think - somebody who is no longer in the NBA, anyway). Does the fact that Williams' is great now show that Sloan was wrong not to start him from the get-go? Or perhaps maybe Sloan knew what he was doing, and Williams became better by having to work out the system?
This is amazingly typical. A player plays well, the team does well, but still it provides "evidence" that Nate's an idiot.