You're setting up a false dichotomy between a game where critical hits kill fully healthy PCs and a game where all combats are clearly going to be victories for the PCs. There's space in between in a well designed game. Space where bad decisions kill PCs, instead of pure, unavoidable, bad luck.
If the game is designed so that it is presumed that you will occasionally enter combat, then single critical hit deaths for fully healthy PCs mean that your PC can die even when you have made no mistakes. That's bad design. And the quote I included illustrates it.
You're presuming, in the material I quoted, that the PCs can meaningfully affect the odds by doing things like exploring, negotiating, planning, etc. But unless they completely stop the enemy from making attack rolls, they can't actually eliminate the chance of a random crit death for a fully healthy PC. At best they can slightly mitigate it.
What actually happens in a system with random crit deaths for fully healthy PCs is that, instead of exploring and planning being things you do to improve your chances at combat, exploring and planning become things you do to AVOID combat. Combat becomes a punishment for having failed in the exploring/planning/negotiating stage.
And that's poor design in a game that presumes combat will happen with some regularity, and which gives PCs all kinds of abilities to do interesting things in battle.