Previously, we had a discussion about lifelink, and I came to the conclusion that if lifelink was to be granted to non-creatures, that non-creature spell would need to be rather special, so that no one had anything to complain about. I don't know if this spell in Magic 20XX covers that ground, or makes me a hypocrite.

This has the potential to be an extraordinary amount of life gain. I never caught anyone use it, though, and it hung back in packs. Maybe it's too risky, or someone needed to see it work once in action. Or maybe no one built a solid red-white deck. I'd say 'Probably needs to increase in casting cost', but I guess playtesting said otherwise.

I'd have pegged this as a rare - since it's a pretty unusual effect. Certainly hits the "Oh wow" factor for me though.

It's probably at least as good as Reverse Damage but I guess if your playtests showed it was fair at this cost; that's fine. Certainly it doesn't need to cost a vast amount; after all, you'll probably win by killing your opponent before you gain too much life from it.

This seems a good use of lifegain on spells, its obviously so much more elegant than spelling it out.

It looks ridiculously good with earthquake effects, but i dont know if you can make a deck to abuse that in practice

This indeed looks pretty cool, but I can see why it didn't get picked highly. It is after all "only" lifegain.