Class => Job => ???

[Ametrine]

This is all on the nose.

Tanaka decided FFXIV needed it's own version of the job system with totally unique jobs in their attempts to separate FFXIV from FFXI. When Yoshi-P stepped in, these new classes not aligning with the beloved jobs of FF canon was one of the first bits of 1.0 weirdness brought up.

So, they just looked at which class could most easily be "upgraded" into a FF job and went from there. In fact, arcanist was in 1.0 as a not yet implemented class option. I guess they felt like this trend was required in 2.x as they made Rouge/Ninja the same way way, breaking the hearts of players dreaming of a Theif class the world over.

Just, please, based Yoshi-P, make Geomancer playable in 5.0 or earlier. Please.

[dinnertime]

Conjurer and Thaumaturge

Conjurer had Protect, Shell and healing spells in 1.0 so it fits for them to evolve into White Mage, and in most cases WHM can use some element-aspected spells. Not sure if they were referred to as healers in the 1.0 lore though.
But yeah, how Thaumaturge becomes Black Mage is a bit of a stretch when Conjurer had all the element-aspected spells at 1.0 launch but it's a thing of the past now. It's just aesthetic though.

Summoner and Scholar

For Summoner...yeah I feel like they should rewrite the lore to unrestrict the variety for summoning, but before that they have to give the job at least Leviathan, Ramuh and Shiva summons.

And yeah, Scholar is more of a war tactician and medic actually than a literal bookworm. It makes it interesting in my opinion.

Thancred still calls himself a Bard

I think he's referring to him being just a bard and not Bard as a job.

I can't say anything about Arcanist or Machinist though since their lore didn't interest me that much to remember, but I should read on the lore of every job to refresh my memory.