SpaceX Falcon 9 Launch | Starlink Mission - Replacing first stage

MECO happens at Mach 8 at about 40 miles up. To replace the first stage with a winged stage that breathes air and lands, one would have to design an airplane that can fly as fast and as high. On the plus side, you would not need LOX, but you would have to refuel before kicking into Mach flight.