FACT: You can't improve at chess

This is going to be a very controversial post - but I strongly believe that once someone has a basic understanding of the game (knowing all the opening variations, basic strategies etc..) it's almost impossible to improve based on practice. I think we all have a natural ability that will dictate our skill level. It's why we see little kids rated as grandmasters but players who have put 20+ years in still struggle at 1500
This is why you see that majority of players, who have played for over 5 years ALWAYS hover around the same rating. You would think after 5 years of consistent practice the rating would gradually increase? 
Every single graph I've looked at at long term players is within 200 rating points. I.e. if someone is rated 1900 they will have hovered between 1800-2000 for their entire careers. It makes me believe chess is based on genetic intelligence you're born with and nothing more. Yes you can sharpen your skill but you're not going to go from struggling at 1000 to 2500 in 10 years.
I know the majority of you are thinking "what an idiot of course you can improve" - Show me a graph of a player who has consistently improved over time. It doesn't exist. It's usually rapid increase or decrease at the beginning then just hovering around a rating forever. Give me a player profile graph and show me slow, long term improvement

IMO its not skill but mental agility that limits improvement. Skill is moving pieces based on rules; mental agility is the ability to constantly reconfigure patterns of play based on creativity and mental agility to see possibilities.

Its a question I have asked as well - practice, in my opinion, has only made me faster at recognizing patterns. Improvement comes not in WHAT you see/think but HOW you see/think, and that, IMO again, IS limited by mental agility.