As the above poster has said, this is an deeply invalid conclusion. Let me give some examples.
No matter how good of a tank you are, you will not survive your co-tank choosing not to tank-swap on many fights, at least not multiple times (when your invul is gone; on fight where the buster applies a debuff, sometimes not even then). No matter how good of a Paladin you are, you're not going to be solo-tanking and solo-healing through any of the Savage fights.
No matter how good of a healer you are, you can't keep someone up through an avoidable 1-shot. No matter how good you are at rez'ing people, every time someone screws a mechanic and dies instantly, you lose DPS. No matter how good you are at green DPS, you will not make up for a DPS operating at half efficency or worse. Both of the preceding sentences mean the best healer in the world isn't getting through fights that have an enrage with the worst DPS in the world.
No matter how good of a DPS you are, you cannot take up healing or tanking the party through through EX+ and even some normal content. You be the best DPS on a given job in the world, and still wind up in a party that cannot make the enrage.
No matter what job or role you are, if someone else just can't perform Black Smokers in E3S, the entire party will wipe. There are many other fights with mechanics like this, either via death or debuffs which doom the party.
Furthermore, it isn't just about clearing the fight for some people, esp. those trying to just get their dailies done. It's about finishing the content in a reasonable amount of time and with minimal frustration.
For the second part, while time and experience usually produce improvement, it isn't a guarantee. One of the first players to get to Inheritor rank in Halo Reach, which required an absurd amount of time played to reach, was notably terrible at the game. People who don't realize they have room to improve will get better very slowly or not at all.
I've played enough semantics wars in my life to know that's a choice of damnations. Use equality, and you get people making disingenuous arguments about inherent differences in ability and "we can't all be equal" etc.
It's also not the right concept; equity, fairness, means providing, judging, and adjusting according to need. An approach based on equality would try to be blind to differences in current needs and past grievances.
I'm not going to use the wrong term in an attempt to obscure my opinion. Everyone has a worldview and goals, an "agenda" as some might say when trying to affix negative connotations, and I'm proud enough of my views to be honest about them.