Better tell that to the bats, bees, honey gliders, flying squirrels, flying fish, pteradactyls, pteranodons, etc., etc. There is even an arboreal snake that does a pretty good job of simulating flight. With your argument the Wright Brothers shouldn't have been able to fly until they were provided with a Pratt & Whitney Jet Engine and a Boeing airframe.
Better tell that to the bats, bees, honey gliders, flying squirrels, flying fish, pteradactyls, pteranodons, etc., etc.
Irrelevant. We're talking about evolving into a flying bird and a gliding squirrel or lizard is not remotely close to a plausible 90% point for getting to flying bird status. For a bird or anything trying to evolve into a bird to fly, even badly, all features would need to be in place.