Well, it's true I don't play as many high-level games as I do low- to mid-level, but I have played them. In my experience, death at high levels very often comes, as you say, from insta-kill effects...
And the suggestion of letting a person live longer at negative hit points won't impact those effects at all.

Nerf your save against a beholder's death eye, it doesn't matter how many extra HP you've got.
So it fails to fix a problem that might actually exist--the prevalence of instant death effects at high levels--while offering a solution to something that (IME) is
not a problem--death through actual attrition of hit points. And that's a circumstance which I still maintain is often too easily avoided. Not
always of course--I've both killed and lost enough PCs to know that character death does happen--but often.