Well it just so happened that the reviewers were people who knew fine dining, and the awards DID drive business to restaurants.

So it could have been a joke from the start, but accidentally became a prestigious award, that happens to be becoming a joke.

The flaw in your reasoning is that you are assuming that everybody whom Michelin could hire is a tire expert. Michelin is just as capable of hiring food critics. They hired eminent critics and experts to write travel guides, and when people realised that, "hey, their reviews are actually accurate, because they have incredibly strict criteria on which to base their rating system", people understandably began to build trust in that system. The fact that Michelin still doesn't seem to have sold out is why there's such longevity to it: they have no reason to make their criteria any less strict (quite the opposite, and I've never been to a restaurant that was even mentioned in the Michelin guide that wasn't some of the best food I've ever experienced). The title of most eminent food review is theirs to lose, so they can't afford to betray that, especially in a world with increasingly-ready access to information. There's nothing that can outmatch the brand loyalty and name recognition.