I think we have to consider not just "fair" I mean is it fair that we have some monsters count as multiple CS? I think fair shouldn't even come into the argument but rather how will this impact junglers and the game?
I think it's a cool idea, but at the same time, it synergizes well with the Aftershock keystone which is a bit too strong on certain junglers. So how would this impact those junglers? I'm an Amumu main and definitely thinking about him as an example.
I like the idea of moving the game towards more strategic battle scenarios, but it has to work across the board rather than in individual spots that might require counterbalance. I know Amumu has traditionally been seen as a bit weak, but his stats lately really say otherwise. He's a strong tank or as AP.
But a gain in either direction will push him over. We need others to match him in the tank realm without buffing him. I think he's where we want all tanks to be. Assassins too actually. The string engage, no disengage is an important part of what keeps him balanced and fair.
As an example there are top amumu players as tank, as AP, and as people that play both. The ones that okay both have the highest win rate and best stats proving that he can play assassin mage or tank right now, and which he decides on has a big impact based on team comp.
People say there are no good tanks right now, but amumu situationally is a really good one, and in poor situations is still pretty strong.
So do we really want to start messing with a rune that synergizes so well with its keystone? Or rather look at how we can transfer that power into individual tanks that need it? I prefer the latter with small base HP growth buffs for example. This way we can more specifically target overall games impact.
Also, keep in mind heath synergizes with defense and defense with health, so this would increase the inherent value of high armor or mres items like frozen heart. Was it is a rushed frozen heart is a lot stronger than most realize.