This is great, but it's also a single anecdote. Clearly there are people for whom it works out.
The real test is whether you, or someone using your identical methodology, can repeat it.
Especially if you either find a niche technology with bad documentation or a widespread technology with a bad problem, there's a lot of cash in it. I think the Meteor.js authors talked about pulling in $500k
As the saying goes, the plural of anecdote isn't data. I think the point here is that it's very much dependent on the niche and that your and other successful authors are atypical and doing better than most self published authors.
Whether or not that's true, I don't know. Maybe you are completely right. But we won't know from some anecdotes.
http://blog.danwin.com/don-t-forget-the-plural-of-anecdote-i...
If you have suspect data, throwing more suspect data in the pile doesn't make it less suspect. Anecdotal evidence is just that: unproven possibly suspect data. Having more of these doesn't make it any more reliable.
So regardless of the original quote, I stand by the plural of anecdote not being data.
I agree that experimental data is preferred, but sometimes observational data is all we've got, right?
It is not, however, good enough to treat as fact or make any general inferences from because it's not very strong evidence.
The GP point is that publishers also cap the upside of the deal. While self publishing, if you win the unicorn lottery, you'll get an unicorn sized reward.
Then, when you consider the hundreds of thousands or millions of people who tried selling ebooks without buying your book to help them, it's clear that the OP had very atypical results.