Enemies capabilities have little to do with job diversity? Except in the case where their specific mechanics completely ruins a jobs performance.
You specifically pointed out me being able to multibox all(not most, ty) of FFXI's content as if it were some sort of problem with FFXI. I have no doubt I could also multibox all of FFXIV's content, were there any reason to actually play it. Chinese RMT can kill everything in FFXIV, they haven't gotten to master trials here after 1.5 years and barely made it to aeonics with SMN ***.
YOU brought up the topic of difficulty, I just educated you on it.
Please, explain to me how vastly different all the 2h melee DPS play.
It's not that they play different, it's that they have differences. Even things as simple as the circles create more variety. By having different jobs excel in different situations, however mildly, you make the jobs feel unique and give the player a sense of association with their preferred jobs.
For a monster like Albumen with heavy debuffs, I would prefer SAM for kendatsuba gear or RUN for inherent m.eva. For a monster that's primarily speed based like WoC, I would prefer WAR for their obscene burst damage with 1hr. For B&W master trial, RNG gets to sit outside aoe and keep their damage up. If a spare stun is needed and the monster isn't particularly evasive to magic, DRK can fill that role while contributing adequate damage. For something like warder of temperance or unafraid of the dark, damage types become important and jobs with powerful secondary weapon options become more appealing. If, for whatever reason, a GEO is unavailable.. angon(DRG) with light shotted dia(COR) will still allow melee to cap ratio.
It's not that the jobs play or are extremely different, it's that the differences they do have are apparent enough to create preferences for certain jobs. It encourages the player to be diverse, and rewards players who favor specific DPS by giving them situations where their DPS will shine.
In FFXIV, you will almost never choose a specific DPS for a specific fight because of that DPS's merits. Everything is built to be interchangable, which is great for inclusion and feel-good factor, but it's pretty obviously a dumbing down.