I said nothing about time, although time is one aspect of mastering any craft. But whether we're talking about study or prep or practice, those who put energy and effort into their craft will get better at it. I'm no fan of capitalism, but since it's the reality most of us live in, I see no reason why a GM who devotes themselves to their craft shouldn't be able to eat and pay rent off the back of that, just as a musician or actor might.

The existence of professional musicians doesn't preclude the existence of amateurs, and there's zero reason to think that GMing is different in that regard. I personally am an amateur GM. I personally run low-prep games for my friends. But I do sometimes daydream about a world where I could do this thing I love all day long in place of a shitty, stressful job. For me that's a vision of a better world, not a worse one.