the lower the altitude the higher the drag and the more thrust required. The reason we normally cruise at 30k and above is simple - the reduction in drag means less fuel (energy) is required to fly a given distance...That is simple physics
The simple physics do not support the statement. Max L/D gives range speed at fixed angle of attack, drag, and therefore thrust required, regardless of altitude because as density decreases you have to speed up to get the lift back. End result is lift and drag back to where they started. As TAS is higher, power required is also now higher. Nonetheless you get there in a shorter time proportional to the extra power. Therefore, in terms of
drag, you require the same energy at all altitudes. But you save time if at high altitude, which is reason enough to do it.
It is when you consider efficiency of the propulsion system that fuel, as opposed to tiime, may be saved by operating higher. The piston engine works most efficiently with a wide open throttle. Therefore best range for a piston is found at the critical altitude where the torque required for range speed can only just be achieved with a wide open throttle. Meanwhile, for a turbine the best TSFC occurs at about 85% RPM and at the tropopause so that is where you want thrust available to equal thrust required for the cruise.
Clearly, if you go to an electric motor then air density does not affect the motor output. However air density will still impact on the thrust available from the prop. Therefore it seems to me that for public transport, electric aircraft will be limited to the 20-30k altitudes unless ducted fans raise the ceiling.