It isn't really an issue, though, because PCs also recover inbetween those fights, etc.
Between all 20 levels a PC will face a few hundred encounters IME, probably 1000-2000 attacks. So, if base damage is 1 or 2 dice, say 2d8+4 for an Ogre.
Normal crit damage for such an attack against a PC would be 4d8+4, so 22 dmg on average. This won't kill any but the lowest level of PCs, even though it might drop other PCs to 0 hp and put them at risk.
Now, allowing an exploding dynamic crit rarely has much impact, and other than low levels still won't result in a kill. At worst, the PC goes to 0 hp as usual.