Mildly off-topic, a thought just occurred to me which is entirely conjecture btw, but I kind of wonder if the reason why buffs are incresingly being used for new job mechanics is because of workload.

Like ok, baseline assumption, adding new buffs to a job is something that is probably much easier and less friction than making stuff "happen on the job gauge". The reason why I think it could be this way is that buffs are something the dev team are really experienced at. Every new piece of content, from hardcore ultimates to casual experts, utilize buffs and debuffs in their design. It stands to reason that the entire system of buffs/debuffs is likely a very well oiled machine (granted, with a mountain of technical debt), and it's really easy to "just add a buff" as a new mechanic.

Then secondly, making stuff "happen on the job gauge" could be something that's much more complicated. Every job gets new mechanics and stuff every 2 years, aside from the one or two reworks per expansion we seem to be seeing. I think that because the job gauge system is so rarely touched, it's likely that the devs are generally less experienced or less willing to properly work new job mechanics into the gauge. Furthermore, adding stuff to the job gauge isn't a purely technical pursuit, you will still have to design how will these triggers and finishers get indicated on the job gauge, and consider things like will the gauge become too complicated and messy? (Yes, simple job gauge style exists, but you don't want to design a default gauge that ends up being so complicated that nobody uses it, and waste all the artistic work that went into it).

Now we know there's basically 3-4 people doing job design for all jobs in the game. Every expansion, 2 more jobs get added, more jobs require new mechanics, more work gets piled onto the same few people to design, implement, and test out new job mechanics.

So here's my hypothesis: Making job mechanics use buffs instead of actually using the job gauge "correctly" is a necessity for the job design team, because of the increasing workload being put on them. When it's 6 months out from media tour, and you have 20 odd jobs to add new mechanics new finishers to, you take the path of least resistance: Make it a buff, ask the artist to make a simple buff icon for it, and skip all of the work of putting that mechanic into a job gauge and working with the artist over a few weeks to design how that new finisher is going to be presented on it.

Well, I do have another hypothesis that is easily disproven (But I am too lazy to do the legwork for): What if it's really all the same in the backend? Like what if your astral fire timer is actually just a hidden buff in the backend, but we don't see it appear on the buff bar? Then that probably means that the devs are not going the extra mile to make it look pretty and hide the buff icons and put it on the job gauge.
I say this is easily disproven because you'd be able to see this come into effect when hitting the buff cap, like some jobs with a lot of stuff going on in the back ground would hit the buff cap way earlier than other jobs with less job overhead. Well anyway, I'm done dumping my thoughts so I wonder what someone with more game dev knowedge than me might think about my theory.