WaterRabbit
Explorer
I would hardly call a 84 to 6 trade-off marginal. If you aren't in a place you can rest, think of all the resources restoring that many HP will cost!!! I don't expect casters to counterspell everything, so in some ways it is a desperation move when it happens, or very strategic to help the party in all kinds of way.
So this is sort of a false premise in my mind. 84 points of damage (in your example) doesn't mean anything if no-one goes down. The 3rd level spell slot when used offensively will prevent a lot more damage than that. Say you are facing 6 mages each going to cast a fireball. You can only stop one with counter spell but you can kill all of them potentially with your own fireball or putting haste on your fighters or or or.
So my point is that counterspell is already very situational and putting more restrictions on it means it will be used less. You might as well go back to 3.x rules at that point where you can counter any spell as long as you have that spell prepared. So fireball can be counter with fireball; horrid wilting can be counter with horrid wilting and so forth.