I don't think you can make that argument at all, and it's wishful thinking to believe it.
There are real genetic differences between people's reaction times, intelligence, logical thinking, etc. Even if you discard all of the genetic differences, your entire wealth of personal experience before you picked up TF2 already mapped out how you'll mentally approach the game and how you approach the process of learning and improving.
There is obviously innate talent in people that defines a skill ceiling - look, for example, at the people who DO train as hard in sports, or practise madly in esports, and still fail. They might even train and play better, analyse the game better, but still lose. The same can be applied to sports, where athletes train all their lives in identical conditions and can lose out to training partners or w/e who have better natural ability.
Work and the refinement you achieve hones your natural ability and brings it close to your personal ceiling, but you don't have the ability to be the next Usain Bolt if you just try hard enough. And to genuinely tell people they do is disingenuous and very hard to believe.
In terms of TF2, most people I think should be able to achieve an Invite level or w/e reasonable level you want to set. There's a small playerbase and it's not that hard even if you have little natural talent.
Also your final point is largely bull too; loads of players have not improved since 2014, or are just as good as they were, or have gotten worse, even if some are better. Also, just because some people improve over time (duh) doesn't mean everybody can improve infinitely until they're the best.