Yeah but how much did your client pay to get that ranking, now many years of blogging, etc. You have to take into account all the people who also tried but failed. One successful example does not prove it's worthwhile.
Obviously, though, the rewards in the former are far lower. But, it certainly means it's worth trying one article to see how big the rewards are. Basically just search like a user would, and see if the results are satisfying. If none are, then it should be straightforward to rank #1 or near the top.
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.