Who says that this spell is Metamorphosis? You even say there is possibility of working around this one ability. This is a clearly new spell I'm talking about that simply retains a 'look', but completely different mechanics and theme. Do you also believe that Demon Hunters would need to retain a Warcraft 3 spell with absolutely no change from the original source?
Because Blizzard can change their priorities on a heartbeat for any given reason. They can set out to accomplish any set path of design, but it can also bend and break at will if faced with something as trivial as 'popularity'. Again, I bring the example of Blood Elves in the Horde. Although we saw the Blood Elves leaving the Alliance in TFT, there was no indication they had any more of a stronger bond with the Horde, or that they would join up with them. The same can be said of the Forsaken. The fact that Blood Elves joined the Horde was motivated by adding a 'pretty' race to balance out the high volume of players who want to play as a good looking female character.
Another big example is Paladins and Shamans opened up to Horde and Alliance. Do you think this was motivated by design? No, it was motivated by balance, since it was much too difficult to maintain the asymmetrical design.
So there is evidence of Blizzard breaking patterns that were formerly established. These are clear 'retcons' of established design. Why would you believe they would adhere to anything concrete when we have been shown throughout all of WoW's development that any design decision can be added, changed and taken back?