And forgive me for the bias, but it seems very difficult for young people to wrap their head around the idea that improvement is not always possible. When someone is young, they literally improve at everything, even month to month... everything, even their height lol.
So I can understand how the idea of non-improvement can be literally incomprehensible. I understand from a personal standpoint too, as years ago I stubbornly argued the other side of this with an adult. "If practice doesn't yield improvement, then they must have practiced incorrectly!" heh.
Based on MY OBSERVATIONS, which you can ofc ignore, these players are just not studying correctly. The best ways of training involve active learning, many slow games, and actively trying to hardwire your thinking process, incorporating your superficial knowledge.
I have met people who have done this, for years, and they have not improved.
They're not stupid people by any means, and they know how to study. I don't know why they don't improve. Maybe the same reason I will not improve to 2500.
I've also met kids (under 14) who are over 2300. They don't seem to work so hard. They have coaches, but work maybe 2-3 hours a day. Some people treat it as a 2nd job, hire multiple coaches, and work 6 hours a day and will not be 2300.
If you start with the premise "anyone can be 2000" then what you conclude from your observations will be biased. I don't know what conditions, if any, would make it possible for most or all people to be 2000. To be honest I haven't met enough people. I assume you haven't either.
So we're left with statistics... like a rating of 2000 is in the 90th percentile, and the obvious fact that not everything is for everybody.