Could an airliner be as efficient lower and slower?
It's important to note that an airliner cruising at 50000 ft (I assume that's what you meant, and that's actually business aircraft territory; airliners are generally below the low 40s).  Is going pretty slow from an indicated airspeed perspective.  At 50000 ft an airplane is going roughly twice its indicated airspeed (it's actual speed through 3D space is twice the dynamic air pressure acting on it - if you stick your hand out, you'll feel the force of a 200 kts acting on your hand, even though you're actually going 400 kt).  So, such an airplane is actually optimized to only fly at 240 kts anyway, if it's cruising at a true airspeed of 480 kts.

What it means is, however you want to optimize for sea level flight, the higher you go the faster you can go in reality for a given fuel burn, whatever the configuration, burning far less fuel per unit of distance in the process.