Often it's not what the prospects become, it's what their value is at the time when they were traded and what the return was. Most were upset that they traded prospects (with perceived value) for declining veterans with back loaded, expensive contracts - when those same prospects potentially could have been used to get more cost controlled, valuable assets.
Also - you're suggesting that everyone knew Syndergaard would become one of the best pitchers in baseball, while nobody should be upset we traded Norris or Hoffman is pretty ignorant.
We also dealt Anthony DeSclafani, who's a good pitcher, yet wasn't listed.