I don't see it. The Blue Mage challenge log is nice, but also very specific and not content most people are interested in tackling. I don't really see the appeal in 'old content but with a team of only Blue Mages.' I don't think content build around a mono-job team comp does anything to compliment a game with a job system at all. It's certainly something, and I do think there is a place for it, but I don't think that alone justifies the existence of a job that literally cannot do anything else worthwhile. Now you can use it to farm bicolor gems I suppose in Shadowbringers zones, which is slightly easier than with real jobs, but beyond that, what else is there? One-and-done spell collection and One-and-done puzzle fights that we don't even get anymore.
At the very least, Blue Mage should be included in PVP, because PVP is already an environment that plays with a different set of tools than PVE. There's 0 reason why Blue Mage couldn't just have its own PVP set and limit break like every other job.
I really cannot bring myself to understand why Beastmaster and Puppetmaster are almost universally seen as jobs that cannot be real jobs, because presumably no one believes the source material for Beastmaster and Puppetmaster is humanly possible to be interpreted in a way that works for FFXIV without hurting the integrity of those jobs' identities despite those identities being defined exclusively by FFXI and nowhere else. And yet that appeal to the sanctity of job integrity got yeeted out the window for Bard who is nothing like any Bard in any other RPG ever made, let alone Final Fantasy, Dancer, a job traditionally associated with random magic and debuffing has not a single unique debuff on its hotbar, Summoner who has never used dark magic or wielded books ever in Final Fantasy, Scholar who was never a healer or a pet job before FFXIV, Sage which has literally nothing in common with any Sage from past Final Fantasies, Warrior which is really just Berserker, or Red Mage who's more accurately a Mystic Knight wearing a Red Mage's outfit. This game's dedication to faithful job identity has less consistency than a McDonald's ice cream machine.