No one should be allowed to queue without a soul stone if applicable.

[Esmoire]

I think this should be a requirement. Either asking for a job stone at least before Titan, or at least 35+, is not asking a lot. I can see why they don't want to hard cap it at 30 flat to give new players some flexibility, though really if you are in your 40s and above without a job stone you should not be allowed to queue at all. At some point I hope they remove job stones on a mechanical level entirely (but keeping them narratively), though right now they would need to change SMN and SCH to make that work since they can equip the same low level tomes.

I assume it's something to do with spaghetti coding, and how classes and jobs are programmed to be seen as "separate classes" rather than one being an extension of the other. (For example: not being able to equip/remove your soulstone mid-dungeon because you're not allowed to "change class" from the one you queued with; also not being able to complete conjurer's unicorn mount quest as a white mage.)

It may not be possible to bar classes from selected duties while allowing them to access others. Either that or they're clinging to the "classes are equally viable as jobs" mindset, but everything else suggests they abandoned that approach long ago.

Nah. PvP already has this limitation. You need to be a non-BLU job to queue. As full jobs by definition start at level 30 at the earliest, that means PvP is essentially asking you to be at least level 30 with a job stone equipped. On a technical level job stones are considered equipment, and equipment is validated before you queue. That's why you can't queue for something where your ilevel is too low.