Am I that naive to assume that if there were no traffic control at all, flying down the "middle of the road" would not result in a collision IF both aircraft had TCAS on?
If TCAS were "prime reliable", never failed, then you might come to that conclusion.
But competent ATC plus TCAS gives an additional layer of Swiss cheese, less likely the holes will line up.
SLOP adds yet another layer, permitting the system to tolerate some degradation in one of the first two. I fail to see why ALL layers are not utilized.