Them being a failure doesn't compute with the very present fact of Blizzard continuing their development of them. Like it or not, but they don't spend time and money developing features that they do not see players login for (MAU says hi).
And the customization for some of the Allied races = better than for the original race. Some are a miss (Nightborne), others clearly got far more dedication and development put into them (Void Elves, Kul Tiran, Dark Iron, Male Zandalari).
As much as I dislike the cynical mechanics behind unlocking them as of BfA's start, it's safe to assume that Allied races will be coming back in the future as a concept now and then. The hype surrounding them alone is worth banking on.