Was the store's cut really lower back in the boxes-on-shelves days?
A store literally provided a box on a shelf for browsing and brought in customers to browse. The digital "stores" don't offer much in that respect.
It doesn't perform the same hard editorial winnowing as brick and mortar, no, and in a digital world it's hard to see why it should. The long tail has fans too and they were terribly served by the old system. But it's hardly fair to call Steam marketing "nothing".
Edit to add: the digital stores also provide an update channel, something which had no real equivalent back in the brick and mortar world. That has real business value.