It's definitely a possibility, especially seeng the whole development with the Nightborne. Blizzard is going at great lengths to seed them as a potential sub-race for the Blood Elves, and they are not doing such things for no reason. Can't say I like this possibility, while I do not have a problem with the High Elves still existing in some (read limited) capacity, the Kaldorei desperately need more development, especially as an Alliance race, and I am afraid that having playable High Elves, even as a sub-race, will only jeopardize that. But since I have no need to be a jerk over this, if this happens, well good for you people who want to play them.
We heard the exact same arguments when people were dismissive of the Warlords of Draenor leaks. Tell me Messrs. "We get extremely defensive the moment any possibility of the High Elves being playable gets mentioned", how did that turn out?