I don't see the 3D feature as a failure. It's just too early. When one 3D TV standard is reached and is even somewhat mainstream, I bet we'll see more 3D phones. 3D is just evolving too fast right now for anything to really catch hold. Give it a few years, then we might see more 3D smartphones.
As for the Evo 3D itself being discontinued, I bet it has more to do with the Evo 3D being able to handle the 800 MHz band that EarlyMon was talking about in the ICS thread. I bet Sprint really just wants to discontinue all of the older handsets, and people who can't get the new bands would upgrade (probably) if it meant they could get higher speeds. Where as the Evo 3D will still work and people wouldn't have to buy the latest/greatest, and thus not paying down their cost for bringing in new phones.