Moreover the advantage of Mode S radar (ground-initiated transactions) is that you're using a self-coherent system: the radar knows what it sent and what it is supposed to receive, and when, in a narrow beam, etc.

It makes lots of spoofing attacks unpractical. Where as relying on GNSS exposes to a whole bunch of 'constellation replay' or fake-adsb-telegrams attacks. Mode S lacks authentication, but there's no reason it couldn't be tacked on one day (through e.g. the already available 'advanced' data-link capabilities or the many available 'gicb' registers...). Even light session-authentication would break most of the remaining spoofing attacks (and most swap problems) on ground-initiated Mode S radar.

I'd further refine "aircraft surveillance" to "air traffic control".

On the defense side ADS-B can provide a baseline air picture of cooperative traffic, but does not otherwise help with detecting non-cooperative aircraft.