How do Demon Hunters have no lore? Have you played Warcraft 3 or the Frozen Throne? Have you done the Demon Hunter-centric quests in TBC and Cataclysm? There is ample lore available, and this includes the Ritual blinding and training to become one.The assumption that DHs have no connection to Warlocks is unwarranted. They may have a link. They may not. There is no justification to state either outcome as a fact. DHs have no lore, no history, no traditions. Their connection, or lack thereof, to Warlocks is unknown.
This comes down to a very basic question
Why include a Demon Hunter spec if we remove all association with Demon Hunters.
This isn't about wanting a Warlock melee spec. This isn't about wanting Demon Hunter gameplay mechanics. It's about wanting to play a Demon Hunter. If you remove all association with previous lore, then what is the point in calling it a Demon Hunter in the first place? You achieve nothing by using the name, because it has no association with the very Hero Class that people want to play. The only thing you have similar are spell names, which frankly has no bearing on anything. Mages have Frost Nova and Frost armor, Lich spells. You aren't going to call Frost spec Mages a Lich now are you?
There is no lore of what a BattleMage is. It's a class that has no pre-existing lore, so Blizzard is completely free to create any lore they need to support such a class. My parallel is if they took your Battle Mage idea and called it 'Death Knight'. Do you believe this would not create any identity issues?Who is to say that a Demon Hunter is not simply a Warlock who chose to focus his warlock abilities on melee rather than casting?...just like a BattleMage and Mages.
TLDR; If you want Warlocks to become Demon Hunters using new lore that ignores everything we knew about them, then why bother calling them Demon Hunters? What is the reason why people want playable Demon Hunters? It's because they want to play as the ritualistically blinded, fel-magic using badass as depicted from Warcraft 3.