I think when you consider what we got for Heyward (Shelby - not Dansby/Ender)... then it's clear the other teams did better with their assets than what Atl did.
Like I've said many times - the Shelby for Dansby/Ender trade kept this rebuild from being a bit disastrous.
People act like the Heyward for Miller trade was some slam dunk because Miller turned out good. They conveniently forget that Miller was considered extremely risky at the time, and trades shouldn't be judged based on hindsight. Trades should be judged based on the process used, and the data available at the time of the trade.
At the time of the trade, Miller was another one of those frustrating "stuff" pitchers whose results didn't match the talent, and everyone was waiting for him to "figure it out". The Braves took that chance, and the Cardinals gave up that chance for an established star who would help them win immediately. Heyward did exactly that, posting arguably the best season of his career. Miller seemed to "figure it out" as far as casual fans were concerned, but many people argued it was a mirage based on the underlying peripherals (he maintained his usual WHIP, K and BB rates, but was very lucky in suppressing HRs).
In stepped Dave Stewart, the dumbest GM in the modern game. He didn't care that Miller's 3.02 ERA was largely an illusion and due for heavy regression. He just signed Grienke to an insane contract, and was desperate to win now. He was pushing all his chips into the center, and Coppy just so happened to be holding a pair that kept him in the hand when Stewart was ready to do something stupid.
You are correct, without Dave Stewart, the Braves rebuild would be verging on being called a failure. The Braves owe Stewart a job in the FO.