Most champion cyclists aren't tall. What matters isn't power, but rather power to weight ratio.

That depends on the discipline. In general, track sprinters are heavyweights, kings of the mountains are flyweights.

Frame size is actually quite important for fitting a bike to a person. However, it's certainly something that could be varied within a model, just like a shoe has different sizes, or whatever. Multiple frame sizes are available on everything but the cheapest bikes - I had a choice of frame sizes on a standard Schwinn aluminum-framed bike 15 years ago.

Given the choice of 5 high-end bikes and the ability to select a proper size I don't really see a problem.

Taller people also have more person to carry around.

The biggest challenge is aerodynamics. If you're over 6'3" and want to be at all comfortable you're going to present a much larger frontal area than someone shorter.