Things as designed right now are tuned well enough that you don't have glaring issues between the jobs. The closest thing to that in recent memory was BRD and MCH at launch, where people would cringe at having them around since they convinced themselves the group's DPS went down just from having them in the group.Balance between jobs matters regardless of the level of content. Any trends formed at endgame trickle down to the rest (see: WARs being told they couldn't tank past HM Garuda when the issue for WARs was turn 4 of coil). Trying to use gear progression as an excuse to throw balance to the wind outside of raiding is short-sighted and doesn't do anything to help your argument.

And even if you continue to ignore it, I'll reiterate: the type of customization that would work alongside preserving balance is in the form of aesthetics. Spell effects, flavor abilities and replacement abilities would go along way in helping develop job individualism without messing with class balance.
FFXI is a terrible comparison since party leaders could always picked exactly who and what they want.
It's an excellent example of what would happen if the devs went along with you. Most of what happened there was a combination of bad design and human nature. You seem to severely underestimate the latter or are just oblivious to the consequences for some reason.