Can almost anyone become a grandmaster if they devoted their life from age 6? I hope that some stronger players have some insight or that their are some quotations from notable chess players that relate to the question.
If you look only at the number of people that are obviously intelligent and put a lot of effort into their chess, and then look at the tiny fraction that achieve the level of national master or Candidate master (never mind the higher levels of FIDE masgter, International Master and Grandmaster) then it would seem that the answer is no, not everyone can become a Grandmaster.

But this begs the question.  We do not know what made the difference. Was some of that effort and intensity wasted in the wrong sorts of study? Is some mental skill present in those that succeed that could have been developed in anyone if we only knew how? Does luck play a part? [Being in the right place, finding the right coach, stumbling on the right books, scoring highly your first tournament]. Or was it genetic?

The Pólgar experiment tells us nothing. It does not disentangle genetics from environment. László Polgár would have done well to learn something about designing experiments before he started. Then he might have adopted a child or two and compared their chess development to the biological children so as to exclude the genetic influence. Even then the results would not have been conclusive: a handful of subjects do not produce a statistically significant result.

Achieving excellence in chess is no different than achieving excellence in anything. There might be a genetic component. Where you live and are raised makes a difference to your opportunities and the quality of your training. Hard work is mandatory (even if some prodigies would prefer you to believe that it all came to them easily without ever having to work hard - because that is a myth).

What is the answer to your question?  It is simple.  It is the answer to many questions. It is not one that is popular. It is the reason for interminable debates and arguments, for cherry picking and for much opinion dressed as fact.

WE DO NOT KNOW!