I don't think you know how to shade properly. The inherit softness of an airbrush can give good color gradiation.
Depends on the type of gradient you want, what kind of lighting you have, your specific style, etc. etc. I personally haven't actually used an airbrush in years.
You're locking the airbrush to one sole purpose of shadow rendering and dismissing the tool as a "plebs lazy excuse".
Good point, but that's literally the only thing I ever see it used for, save for blending swatches.
Even as a shadow renderer, it can be used very effectively. You're trying to be elitist and say "UGH ONLY USE HARD TOOLS. GOTTA DO IT THE LONG AND ARDUOUS WAY."
Generally taking the easy way out doesn't make for a very nice picture, since you put less effort into it.
Even though the airbrush can render very realistically.
You've said this already. There's more to shading than simple gradients. You have to take into account the possibility of more than one source, how light reflects, the materials being illuminated, etc. etc.
There are plenty of professional artists that use the airbrush tool, I've seen it. It isn't about the tool, its how you use it. The artist makes the tool, not vice versa.
An artist that uses a medium adapts their style to it -- the tool is shaping the artist's style as he shapes how he uses the tool. A sculptor working with hard clay will develop stronger fingers with practice, so that he has more control over how the clay can be molded.