While I agree that job design philosophy does need to change, I disagree with your points in that they are already kinda happening with certain splits within roles, and jobs only being situational will just lead to more player frustration.

Instead, I think jobs just need rules to them, rules that mechanically, and aesthetically make sense for each individual job. I think good recent example is with SAM, and how Tsubame-gaeishi has changed. It used to be that only Iajutsus used at a certain time would proc it, but now almost every cast Iajutsu procs Tsubame. The rule is now every cast weapon skill procs an instant version of itself, it's easy to understand, fun to execute, and affords points of optimization for players to chew on. A theoretical example of how this can change job design is with GNB, and Continuation. It could be made that when you unlock Continuation it unlocks the procs on all cartridge skills. The rule is that cartridge skills proc Continuation. This frees up the leveling slots at level 86, and 96 for other things, it moves gameplay elements to an earlier level, and it means that any future cartridge skills already has its Continuation proc unlocked freeing up design space for things other than Enhanced Continuation traits. A lot of knock on effects just by establishing a simple rule.

I think a problem with jobs now is that they lack these rules, and end up feeling like a mishmash of random mechanics that are in a just good enough, playable state.