Buffing to 75% chances of getting 0 damages would be doubling efficency. (chances of losing 1hp divided by 2), but here, you completely remove this chance, which can't be expressed in term of %
Anyway, I personally think this trait is quite useless, and was surprised to notice this 50% thing nerf when I used it, since good players tend to never get hit. I guess it must have been added after some "abuses" with P-modded red + TaN or things like that, which probably made characters invulnerable too easily.
The only run in which I used survivalist was... an AoHu run. I tanked with a freaking AoHu, even through the mortuary ! It was ridiculous, and made the trait fun... for once. But this trait blocks SoB, so... still "worst trait ever" for me :p
And about your idea, well... I wouldn't argue against a buff, btw, but the overheal is more interesting than the ability to become 100% invulnerable, and nerfing that looks like a bad balance choice