How many of those people are making more than $50k per year at it though?

It’s not “no true Scotsman” if there’s a defined dollar value that makes someone, so to speak, a Scotsman. I mean for all I know you are right and there are plenty who are supporting themselves doing it- but the point is not that writers don’t exist; it is that the number of them who are making a living without some other means of support is way smaller than it should be.