You will still need 'primary' radar surveillance in case a non ADS-B equipped aircraft is flying. I don't know the present number of aircraft equipped with ADS-B but it must be less than 50% when you take into account those aircraft for which it is non mandatory.
Or, as is being done in a number of parts of the world, you make Mode S mandatory and use Wide Area Multilateration (WAM) with no PSR backup. Armenia and Tasmania are a couple of examples that spring to mind.