Aside from such a scheme dominating a particular region and being a potential playground for other anticompetitive practices, nothing about the idea in general

Aside from such a scheme dominating a particular region and being a potential playground for other anticompetitive practices, nothing about the idea in general strikes me as inherently anticompetitive. Insurance cos can still compete on the merits of their networks and, in theory but sadly I think not in practice, these schemes could work in favor of the patients if the insurance company uses their network to balance cost with service quality. A very dominant one could effectively rule a region as a cartel, though.