Blogging is basically just content marketing, which has been proven to work when done well. It is also one of the cheapest ways to market if you have a little time each week to invest into it. I know of at least 10 companies and/or individuals who have used this strategy to build an audience and start selling one product or another with great success and a very limited initial budget.

You are right that one (or ten) success example(s) doesn't prove that this strategy works, but similarly neither does one (or many) failure(s). It depends on the quality of the articles, the audience, and many of other factors, just like paid advertising depends on various factors like where the ad is, what audience you target, cost, etc.

What I can say with certainty is that there are definitely markets where you can get to the front page of Google within a year by just writing a few quality articles every month. You don't have to pay money, use spammy backlink tactics, or anything like that. Whether OP's audience is one of those is unclear, but it seems that he knows his audience reasonably well if he is making $$ from them, so chances are he could get a positive ROI on a blog if he gave it the proper attention.