for autonomous cars - this will not be a problem see
LiDAR Scan Matching Aided Inertial Navigation System in GNSS-Denied Environments. - PubMed - NCBI - sensors ( either lidars or even 'temporal smoothing' from radars in teslas ) provide sm level accuracy just fine in urban environments.
It is open areas which does not allow sm accuracy with these tools alone - and here sm accuracy can be provided by augmented gps
And this accuracy can be provided not only by dispersed stations - ex Japan launches a space system to achieve sm level accuracy from gps over all Japan in few years ( by 2019 )
so gps, ins, lidars and radars are complimentary and it is ok that there are more ways to achieve better positioning of autonomous cars in most situations.