I know, but no set can do that perma for all damage types. Some can do it for a couple types, some can do it for all types sometimes but not all the time.
So, in principle I am all for giving Regen 100% -Regen Resist, it just should be in a click, like Instant Healing.
Keep in mind, the -Regen Res in Fast Healing is slottable, so even just increasing it somewhat would help a lot.
How does -Regen Resist work anyway? If you did have 100% res and you were hit with a 500% Regen Debuff, what happens?