Let's try the situation from a different perspective:
"What effect do you think a mid-air with a commercial aircraft flying in uncontrolled airspace, where it is well known that not all traffic cannot be seen, is going to have on commercial operations?"
Just look around europe for your answer. For example in germany IFR traffic is not allowed to fly in uncontrolled airspace (except approaches in Class F, but only one IFR traffic at a time). Result, the controlled airspace gets changed so that all IFR traffic can safely fly within its boundary and further protected so that it is all transponder mandatory (which means since march this year mode s mandatory) airspace.
That leads to many nasty problems especially for glider pilots as the airspace available to them gets reduced every single year.