Massive damage is a variant rule, but I do agree that's probably a good measurement to determine what might cause an injury. After all crits can still roll low, but taking 48 points of damaged when your max HP is 60 is not going to leave you looking pretty. Not to mention things that can cause massive damage is slightly rarer than just a bunch of weak attacks getting lucky with a nat 20.

Also instant death is basically if you take more than double your HP in damage. For example if I only have 20 HP and take 40, that's instant death. And that's also still a variant rule.