The main two types of flier before quadcopters were fixed-wing model airplanes, and helicopters. They are both riskier to use than quadcopters: fixed wings can't be stationary and need a large area to fly around, while helicopters have large dangerous blades. This is why either of these are less attractive to the mass market.

On the flip-side, I think converting a quadcopter to use an engine is expensive because controlling 4 blades at once is not as simple as with 4 electric motors. Either they need to have 4 gas engines (expensive, not as responsive as electrics) or they need to have a variable pitch mechanism (now you need an intricate/expensive mechanical system of levers and servos to transfer the control).

It's a scaled up version of his stingray.

The issue is that stability and control is easier to do with 4 blades rather than with one that has a swash plate.