Except that's exactly what it is. I have, many times, proposed taking
reasonable steps to address problems; putting in effort to
help prevent issues rather than totally eliminate them; etc. And nearly every time,
someone tells me there's no point, you can't fix the problem completely.
The argument is absolutely
the perfect solution fallacy: after all, you yourself just said, "no system is GM-proof, and I can’t control anyone’s behavior but my own." Since we cannot GM-proof the system, we shouldn't even try to make better rules that are harder to mess up
without ill will--but simply with mistaken beliefs, wrongheaded efforts, or accidental perverse incentives, etc.
It's not just in how it
teaches GMs. You can, in fact, actually design rules which make abuse
less likely. They cannot stop intentional malfeasance. You cannot use a system to protect players against a determinedly
bad GM.
But you can do a LOT to protect players (and indeed, the GMs themselves!) from accidental, well-intentioned faults or damaging errors. Better rules design does, in fact, have ways to address some of these things. But because we all recognize that it is impossible to "GM-proof" a system, people trot out the perfect solution fallacy as the justification for never even bothering to think about how we might build up resilience against mediocrity and well-intentioned unwise ideas.