Or rather, that development of the plane was waiting on the engine. The Wright brothers did all of their iteration and experimentation on relatively flat terrain flying close to the ground, which is how they were able to survive at least eight major crashes (and probably more minor ones.) Without an engine this kind of iterative experimentation seems almost impossible: you’d need to launch from a height, which means the first minor failure probably kills you and destroys the prototype.

And of course even if you survive the iterative development process, all you’ve got is a (largely) useless glider until the engine comes along and you can commercialize it.