Right, but isn't a lot of that necessary radio aircraft-to-aircraft, like ADS-B? Obviously not ideal, but I thought IFR traffic could feasibly maintain separation without ATC help.

ADS-B can't legally be used to maintain separation. It's only approved for use as an aid to awareness for additional safety.

Could you in an emergency, if you had no other options? Yes. But ATC can't legally give you a clearance that was dependent on using ADS-B for separation.

(Similarly, if there's a potential traffic conflict, and you tell ATC you see the other traffic on your ADS-B display, they just have to disregard that statement and assume that you can't see the other aircraft at all.)