I'm going to go for the easy answer, I am aware of that, but the longer I think about it and the more context I add to the issue, the more it is the only thing that makes sense.
The chess challenge doesn't adjust. It has been designed to be played by Ron, Harry and Hermione only, and took their relative skills and importance regarding the following challenges into account. You are overthinking it.
The whole existence of these challenges is flawed. Their purpose is to keep intruders/robbers away by testing the abilities of anybody is trying to reach the stone in various domains. It even bothered me when I first read it when I was 10 years old :
- If you want to restrict access to weak persons, make it that only those who beat challenges can access.
- If you want to keep any stranger away, keep the key(s) in your pocket.
In that case, the stone would have been perfectly safe if Dumbledore had kept the token necessary to access the stone in his pocket.
One might argue that it would have been impossible for anyone else to access the stone if needed and if Dumbledore was unable to open the door/give that person the key.
For one, that situation is very unlikely to happen, and it still doesn't justify a process that requires for anyone, whatever his/her intentions are, to undergo life threatening challenges just to access the stone. It's the opposite of defense/protection since it puts the owners of the stone and those who would want to steal it in the same position: forced to use their skills to be deemed worthy of recovering or stealing the stone.
It's as if instead of protecting the stone, they'd put their brightest minds on designing the perfect way to put the Philosopher Stone back into the game, waiting as a reward for whoever completes the challenges. It puzzled me as a ten years old, and it still does. The only purpose of this protection process is to show a confrontation between Harry and his friends and Voldemort and his servant, but while respecting some requirements:
- The confrontation cannot be direct, since the heroes are only 11 years old first years. They lack both experience, knowledge and skill and any direct confrontation would have been both quick and fatal, no entertainment there.
- The confrontation must still involve some risk. It's the end of the first book's thread of action, right before the ending. It's the final boss and therefore must be challenging.
- It must make the heroes rely on their specific skills in order to build in the mind of the reader the idea that the three of them are complementary and need each other in order to succeed.
The third point is the reason why there is a chess challenge.
Harry Potter is emphasised throughout the whole seven books. He is the best at Quidditch, his childhood was a Roald Dahl novel, he is brave and skilled.
Hermione's forte is knowledge and logic. That's lucky, because there is never enough knowledge. That way, help can come to Harry in the form of knowledge at any time necessary, that's what Hermione is, a knowledge trump card.
Ron, in fact, is not needed, his purpose is to complete Harry's family. He brings his whole family into the story, that quickly adopts Harry, and is like a brother to him. Since he isn't needed in the confrontation, it had to integrate an element that would make his presence mandatory, so that Ron can shine once in the freaking book.
That's why chess. The author gave us hints at some point that Ron is a strong chess player and then you forget about it, believing it's trivia, because that's the first book of the saga and you aren't yet accustomed to that technique. And then, the only way to continue to hunt Voldemort is by winning a chess game against enchanted pieces, and moreover, you have to play as pieces yourself, which means that you can be killed. There is no other way around it, you need Ron.
By being strong enough at strategy to not only beat the magic board but doing so with the only loss not being fatal - I assume here that Ron doesn't die because the piece he's mounted on takes most of the blow and that it was the plan - Ron saves the day and gives a perfect reason for being of no use afterwards: he is knocked out because the price of victory was his own sacrifice, which he paid.
To answer your question, my best bet is that the game doesn't adjust.
- It doesn't adjust in the story because it was designed so that one needs to be good enough at playing chess to be worthy of the Stone - I'm happy Ron's forte wasn't eating contests... - and therefore the magic pieces were strong enough to beat most opponents. A grandmaster would have won as well, but maybe without having to sacrifice himself or a friend.
- It doesn't adjust in a meta perspective because it was designed while having in mind that particular group of three, including Ron and his ability at playing chess, which had been itself introduced before foreshadowing that challenge. That challenge was designed specifically for Ron and two more people that needed to keep going after it.
Please, keep in mind that I'm not claiming that JKR was lazy here - or that she wasn't. That's children literature, while that kind of novel perfectly can be subtle and everything, it doesn't have to.
That's okay, and she got slightly better at this sort of schemes in the subsequent novels, anyway.