You are relying too much on the concept of a generalized mental capacity, which many researchers (maybe even most) would say does not exist. You seem to be assuming that grandmasters are just amazingly smart and they happen to apply those smarts to one particular skill, but what's closer to the truth is that they happen to be particularly amazing at that one skill, in a way that doesn't really correlate strongly with having amazingly high skill in other areas.
If you took the people who would have grown up to be chess grandmasters, intercepted them just before they started spending significant amounts of time studying chess, and had them instead spend all that time studying programming or physics or something, how do you think they would rank up?
You seem to be arguing against the idea, "A chess grandmaster is a genius and therefore can walk into Google and immediately start doing more and better work than most of their senior programmers". I don't think anyone believes this (correct me if I'm wrong).
I think a more serious idea is, "A chess grandmaster is a genius and therefore learns faster and has a higher performance ceiling and such than most people, and if they spent a couple of years learning to program, they could become an entry-level Google programmer, after which they would rise more quickly than most hires, and eventually would outperform most of Google's senior programmers."
By the way, I think most of the best chess players were extreme chess prodigies. (Just looked at Kasparov, Karpov, Shirov, Kramnik, Anand, and Carlsen's Wiki pages; all but Kramnik had the year listed, and they all became grandmasters around age 17-19, except Carlsen, who was around 14. Kramnik's page mentioned winning a gold medal for the Russian team at age 16, and that he wasn't a grandmaster when selected for the team and this was unusual.) I think this is consistent with them being highly gifted children, who choose to spend their time doing chess.
>I don't think anyone believes this (correct me if I'm wrong).
While not exactly what you wrote, people do think that someone is a genius at some subject can become a genius at another area with less work than it took for someone in either field to originally become a genius at that field, especially society groups the areas together (so sports star becoming master programmer is far less likely to be believed than chess master becoming master programmer).
