However, in order for that to have happened, we needed to have our brains manipulated a very specific way as children, when plasticity was at its highest, into learning how to think the way that best fits with being a chess GM. Since that didn't happen, our minds have settled into patterns of thinking that are very hard to now undo or rebuild, and likely not as well suited for chess.
Could we tear down how we think about problems at a fundamental level and rebuild ourselves to think like GMs? Yeah, probably. Could we do this while maintaining any amount of day job, social life, or literally any other aspect of a natural human life? No, probably not. Changing not just your opinion on a topic, but literally how you think, seems pretty insanely hard to do after childhood/adolescence.
I don't love the phrase "mental capacity", maybe "mental configuration" is more accurate.
Is it really true everyone has the same mental potential?
For the specific case of a chess GM, it could be many people have the potential. Or maybe not.
There are some easy tests to quickly observe and compare mental traits between people.
For example, some people can juggle many distinct numbers in their head at once. Their brain is like a computer memory. Personally I start to fall off around 4 variables when doing mental math and don't believe I can increase that significantly. Do I have (or ever had) the same potential as someone who can juggle 50 variables in their head?