Imaro
Legend
This is the point.
The only possible way a player can succeed in dnd if the GM lets them succeed. The only reason they won a fight is because GM decided not to bring reinforcements. Didn't, but could. Could, but didn't. Ooh but she needs a narrative justification! Yeah, those cost nothing. I'm not a particularly creative person, but I can create a logical explanation for a new pack of goblins showing up. Or a lich. Or a tarrasque. Or an orbital ion cannon locking on the PCs and evaporating everything within a kilometre of them into fine dust.
Yeah, the GM can show mercy (aka being unwilling to crush PCs underfoot) and decide not to field monsters/obstacles/environmental hazards/whatever that completely shut the players down.
Yeah, we can say that a "decent GM" shows mercy.
Doesn't change much, though.
Doesn't encounter budget/CR address this? I mean it gives a measuring stick for the different types of encounters and thus constraints that skilled play can be judged upon. How well does someone deal with an easy vs. hard vs. deadly encounter. There are even guidelines for what is expected in an "adventuring day". Yes these can be ignored but then so can the constraints in any other game.