The importance of GEO has more to do with how they dealt with 100+ content than the job itself. Since there doesn't seem to be any evidence contrary to a linear level-based stat increase system for mobs, (EX: acc+34 required per mob level over yours,) the differences between older support role jobs like BRD/COR/RDM become painfully obvious in direct comparison in higher level content. SE has always had the bad design habit of addressing job mechanic issues with equipment rather than actual job adjustments, so the older jobs (who have primarily static value based buffs) just ended up lagging further and further behind as player/mob stats continuously climbed while their unchanged mechanics (with occasional small boosts from equipment) provided less and less overall benefit.
GEO's bubbles that have direct comparisons/overlap with other supports are percentage based rather than static, so they naturally scale along with player power increases. Once upon a time, Madrigal's/Minuet's static values provided a sizable increase to accuracy/attack when viewed as a percentage of what players were able to attain. Player stats have skyrocketed compared to pre-ilvl days though and the percentage increase that those unchanged static values provide now are much smaller percentages which pale in comparison to what a moderately geared GEO (Dunna) can provide. Toss Idris into the mix and there's no way possible to compete. This is not a flaw with GEO, it's a flaw with the other support jobs.
The modern game is not being designed around GEO. GEO is just designed for the modern game. The older jobs aren't and are suffering for that. This is not an issue of "GEO is Over Powered", its an issue of "Non-GEO support jobs are Under Powered." The solution is not to nerf GEO, it's to buff the other supports to competitive level. If nerfs are needed after that has happened, then we ask for nerfs at that point.
Nerfing GEO with the current state of the game though would be a textbook example of "cutting off your nose to spite your face." It would provide zero benefit to anyone and hinder most groups, so what is the purpose for even asking for nerfs right now?
The old content also did not normally have to account for haste capped melees. ^^;; I have the distinct impression that most players (and possibly the devs) ignore the impact that the increase in TP fed to mobs by delay capped melees with sky-high multiattack rates has had on TP move usage. Even if an old mob had some sort of super nasty AOE, it would take FIVE DDs to match the baseline TP fed by only ONE modern melee w/ 80% delay reduction. That means that they are getting fed TP at 500% of the old rate. And that means that what would have been a 10sec delay between TP moves with 3x DDs pounding away is now a 2sec delay and basically firing off back-to-back (and likely to be less than 1sec once multiattack is taken into account.) Would the majority of current gen nasty AOEs be anywhere near as annoying/dangerous if there was actually time for a WHM to clear off at least 1-2 debuffs on everyone between moves?
That was a bit of a tangent, but still leads back to why GEO is dominating the support spot in groups... When we don't have access to any other effective means of defense (in part because the older supports simply can't provide enough to matter much in the upper clvl world,) the one support job that is capable of providing anything worthwhile will of become the default/go-to option over those who can't. Which leads back to "Don't nerf GEO, BUFF other supports to match." It's fine if there are differences in what each job brings to group content, but there is a problem when the choice is not a matter of tactics but basic functionality.