> if all but one of the satellites used sat-to-sat links and only one had a downlink you'd see a substantial reduction of the available bandwidth per satellite.

All the satellites above me have an uplink from a ground station.

Also, satellites above ocean with few people in their footprint can see my uplink but can't use it effectively, because they don't have many users in their footprint (possibly none, because of elevation angle restrictions).

If we have satellite-to-satellite links, then those satellites above ocean can share their bandwidth with satellites above me. Therefore, real system capacity goes up.

Assuming that ground stations are upgraded as the size of the constellation grows-- they're strongly directional. Their throughput is the uplink bandwidth * the spectral efficiency * the number of satellites within LOS of an uplink ground station. Intersatellite links let you use more of this throughput without it going unused and "spoiling"-- that's the main point.