They only call them "Jobs" for nostalgia.
This would all be less confusing if they called them "specializations", which is really what they are. A few bonus abilities.
I played my (job) character at level 50 and it seems like classes are still as useless as ever, solo or not. They should just scrap the system... ugh. It's so confusing for people.
Coming from XI you could apply the same theory for job/subjob of them being "specializations" if you wanted to exaggerate. What you are not taking into account is how much bonus abilities will differentiate Paladin from gladiator as the level cap raises.
Seriously, we don't know how jobs or classes will develop past level 50. Gladiator could drift way more to the DD side as Paladin gets more unique skills for tanking. They didn't have the roadmap for 1-99 for FFXI jobs and how their subjobs work either until time progressed.
The game development doesn't stop at the current cap unless the game tanks and gets shutdown. This game could turn into something pretty different from the launch if it survives 5, 10, 15 years.