A couple of points:
You are assuming that chess MUST be a draw with best play, a contention that has by no means been proved.
If your definition of "good move" changes with the situation on the board, then your original explanation of the term was indeed imprecise.
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All that is necessary is to define a good move as anything that doesn't alter the game-state, since it's impossible to produce a move that alters the game-state positively. So it's precise.
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If chess were ever shown to be a forced win, I think many people would stop playing because it would entirely alter the game and the way it's played. It isn't going to happen, though, so it's an assumption that doesn't require any more proof than already exists. I actually believe that it is completely impossible to arrive at a deductive proof regarding the outcome of chess with best play.
A couple of points:
You are assuming that chess MUST be a draw with best play, a contention that has by no means been proved.
If your definition of "good move" changes with the situation on the board, then your original explanation of the term was indeed imprecise.