That isn't to say no mechanics challenged a caster's ability to maintain their uptime. Even in the final tier you had Cloud of Darkness which, in addition to several mechanics which led to casters needing to traverse a good chunk of the field (tethers, Anti-Air if you got the shit-end of the stick positioning wise, the social distancing squares mechanic). In Red Mage's case specifically, Thancranjit presented issues when he did lightning at 2min in if you were running 2 tank 2 melee because there was hardly enough room to be in to do the melee combo at the burst point. Mechanics like Stormy Horizons were particularly taxing for casters to plan ahead to as well because of the quick nature of the AoEs being dropped out.
Casters worked to get around these problems, but that's their gameplay: to work to get around those problems with either team coordination or planned usage of mobility skills ahead of time. You can argue that the casters have been getting more tools as time goes on, and that's true especially if you look at Summoner who is now so mobile they have a "casting phase."
Where we agree on this topic is that mobility tax is too high. The entirety of the physical ranged and summoner don't need to be the weakest jobs because they can move about with near-impunity. Where we'll have to agree to disagree is in the idea that fights are not designed in such a way where mobility and arena-denial are accounted for; in Ultimates, despite the "Rez Mage" meme, Summoner was always more valuable because of how much more easily it could navigate the field with its (yes, even then) ridiculous level of mobility on a caster. Groups with Summoners could have the Summoner be the Phys Ranged's backup for Liquid Hells. Groups with Summoners didn't need to plan around one of the players being stuck in the mud during Brute/Cruise. Summoners also had to drop next to no uptime at all handling Ifrit baits in UWU, and even baiting during Annihilation, whereas a Red Mage or Black Mage would inevitably run out of options to keep its GCD rolling during those phases.