What even is the purpose of all those non melee/blm jobs?

[Lyth]

...

Who decides which jobs have a 'lower skill ceiling', though? You?

Historically, the jobs which have done the most damage haven't done so because they were technically challenging by any stretch of the imagination. They've done so because they've been arbitrarily given that advantage over everyone else. At which point everyone switches over to that job and claims that it's the most challenging. But they'll drop it like flies once it falls out of favour.

rDPS balance should never be skewed based off of arbitrary, player-based claims of job difficulty. Ever. They will always, always argue this point in favor of whatever lets them do them do more damage on their comfort job for less effort. If you're a good player, you don't need that sort of job-based advantage over everyone else. Your personal skill will be all the unfair advantage that you need.