It suddenly dawned on slow-old me that the meta-game premise for d20 in general is that four CRx vs. 1 CRx is a fair fight (for the four). Why? They will almost certainly win, barring terrible luck synchronising with the one’s bad luck, and lose very little resources. As soon as you look at it the other way, say the PCs are the monsters and the NPCs the heroes, it becomes quite bizarre and truly gamist.
The assumption that many d20 players make is that all their encounters will be within the CR guidelines as presented in the DMG. This automatically prevents sandbox play if the GM adheres to that formula. If party of 4 level 1 characters goes deliberately hunting the great-dragon-sitting-on-treasure-mountain then the CR of the dragon has to be CR5 or less. What that does to the internal-causality of the world is nothing less than defenestrating.
Sure it’s preference and all that but basic logic shows it to be nothing more than an insurance policy against player death. If that’s the meta-game you want when you play why bother with rules of conflict at all, since ultimately the player has to win?