I saw that Lefsetz article. I usually find concepts of "pros" vs. "hobbyists" are passed around by pros to make themselves feel better about their profession becoming more accessible to hobbyists.

I prefer Steven Pressfield's take:

How Pro Are You?
http://www.stevenpressfield.com/2012/12/how-pro-are-you/

I'm in the camp that doesn't believe in luck. Of the handful of full-time "professional" musicians I know, all of them have a common trait: perseverance. Not talent, not luck. Just a determination not to stop and to keep looking for the next opportunity.