I'm going to put this into perspective.
Take the Red Mage class for example. Everytime you use a spell, dualcast pops up. Veraero and Verthunder have long cast times, they're meant to be used with dualcast in mind with a quick casting Jolt/II/Impact. In addition you have a melee combo that becomes enchanted when mana levels are high and balanced. I just described the basics of the class. After this comes the normal DPS stuff, do your rotation, avoid aoes, kill adds, do mechanics, and so forth. Finally use utility/roll actions as needed to buff the party, boost your mana, and so forth.
Have I said anything that is beyond what anyone is doing here? No I highly doubt it. Even people here who haven't played the class understand what I'm talking about. Yet I've seen people 'not get it'. I was in a dungeon about 3 days ago where a Red Mage was hard casting verthunder. This wasn't a prepull cast, this was near the end of the fight. The amount of damage to say the least that they were contributing was alot lower than mine to say the least.
What you describe as people taking the game more seriously takes 'they get it' to a whole nother level. Or so at least in your opinion it does. But does it really? These players do the math, do the testing on dummies. And find the BiS gear and materia to get ahead. But other than that. What are they doing that you can't or don't want to do? I know what it is, but suffice to say, knowing how to play the class is not part of it. Make no mistake, they are putting more work in that department than you are.
But are they actually heads and tails better? No. The average player knows to dodge AoEs. The average player knows to deal with adds. The average player knows all the mechanics they need to deal with. Sure they may struggle in a new encounter, but they'll learn it. The difference in performance isn't drastic. Just like some who 'don't get it' will still dodge stuff and try to do their role even if they've got no clue how to do the basics of their class.
The real difference is the organization. Those who play at the top level rely on statics and organization and teamwork of like minded players. If you're saying you're incapable of doing that, I'm going to call BS on that. You're responding to me in a clear and coherent manner. That alone tells us you're totally capable of doing the top end content of this game. You have the basics of your class. You know how to maximize the class through gear. And you're capable of intelligent communication. You're capable of learning mechanics (I assume, you've given me no reason to believe otherwise). What other ability are you lacking? I don't believe you do. And take it as a compliment if you wish.
A desire not to play in the highest content does not equal an incapability of it. Not wishing to adhere to a schedule to do content is NOT a sign of ineptitude.
There IS varying degrees within players who know what they are doing and those who don't. But within those two parameters, the differences are almost negligible. Again only limited by desire or outside influences (like physical handicaps or bad internet), or simply other priorities.