That's not alchemy, that's brewing (part of the cooking profession).
Rogues can also concoct acidic poison bombs, like the warcraft 3 unit, does that make them alchemists?
Survival Hunter old description: "A rugged tracker who favors using animal venom, explosives and traps as deadly weapons." - which, basically, fits what they are now - Headhunters using Spears:
"These cunning warriors are trained from birth to hunt, track and trap the most dangerous beasts in the wilds".
Demonology description from Mists of Pandaria:
"a master of demonic magic who transforms into a demon and compels demonic powers to aid him". how times have changed, ah?
But, you also mentioned Liches, Frost Wyrms, Abominations, Dreadlords, Banshees and other undead units. and the Death Knight wasn't meant to be any of them.
Again, you only mentioned Goblins. now you decide to mention Gnomes. their approaches are quite different. just like there isn't only the Goblin Alchemist way. there is also the Troll Witch Doctor way and the Forsaken Apothecary way.
Yes, i do believe. old Survival was melee as well, don't forget it. and it's not just Hunter discarded abilities, its also mage and shaman. did you believe they could make a Demon Hunter, and give it two specs at that? no one did before legion. everyone was skeptic about it. you focus too much about gameplay. there's the fantasy urge that needs to be satisfied as well, that the hunter does not deliver.