So all of the Ex battles in the game are like this? You're focusing on the current tier, the game is quite a bit bigger than the current tier. Also, you're considering the content as a top 3% player at endgame. A vast proportion of the playerbase do find content challenging on their first run through and many need easier jobs to get through it - so the jobs need to be balanced with that in mind.
BLM might not be that challenging for you, but its a bloody sight more challenging than RDM. You certainly can tank your damage on a BLM and have easier jobs equal or out perform you.
I would also add that knowing mechanics is fundamental to BLM performance due to how it works, even if you are an "average BLM", if you have played content a ton of times before and have memorised the mechanics you are at a distinct advantage. BLM's rotation isn't all that complicated, heck its pretty rythmic and intuitive when you understand what the gauges are doing. The difficulty of BLM is being able to keep that rotation going with your buffs up through mechanics.
What have FireMage and toxics got to do with anything? Difficulty scales between jobs are required for the game to be accessible to people outside of the hardcore crowd, and there has to be a benefit to using more complex jobs. In an MMO this isn't just about personal challenge, but about social aspects too. You might want the extra challenge, but does the rest of the group want a higher risk job in a group when a safer job can do just as well?
Yeah, SMN was made a lot easier and still has a fairly high damage output. But, its kind of an exception right now.
As for BRD, come on? Really? Bard is the exact opposite of a selfish DPS, while their personal DPS might be in the "have nots" their whole thing is making the rest of the group stronger. That balance is kind of fundamental to BRD.