That's its origin, but it's certainly been repurposed today in product design to mean a very real thing, which is that the customer will buy what the customer wants, regardless of whether you think they should or not.

In other words, there's no objective product goodness/badness. Only what the customer wants. In that sense, the customer (market) is always right.

Inexperienced restauranteurs often experience the same shock. You don't cook the food you want to make, or that you think people ought to eat -- you cook the food people want to eat. Otherwise you'll go out of business.

It's created an inappropriate sense of entitlement for many consumers. You're absolutely right that a product will fail if people don't want it, but it's also true that people often don't get the products that they want because there's no way to deliver that product to them and turn a profit. In these cases, the customer is most certainly not always right.

It's a bad slogan that only portrays half of the reality of the situation.