1 - there's plenty of lore and gameplay justifications for that, so it wouldn't break lore;
2 - it would allow us to have night elf and draenei warlocks under the "noble sacrifice to fight the legion" excuse;
3 - it would allow the warlock class to maybe finally get a class hero that doesn't go batshit crazy and has to be put down;
4 - it would give devs room to work on new classes that can't be fit into other existing classes;
so I'm not opposed to demon hunters as a class, but considering that they could exist within warlocks and it would be so positive to the warlock class and for other would-be new classes, I believe making a new class for demon hunters would be sort of stupid.
but let's just wait and see what blizz is gonna do