"Getting acquired and becoming a cost center for another organization is not how I'd define success."
I'd not consider that failure, either; Many things are valuable, yet often defined as "Cost centers". IT Support, for example, is almost always defined as a cost-center, yet it fuels so many parts of the business and can be such an improvement to getting things done it's insane in the modern world not to have it.
Building a technology or product that is only good as an add-on, loss leader, or tech demo isn't shameful. If somebody's willing to pay for it, if you and your investors can get a good return, that's a reasonable measure of success. It's nice when a product is successful on it's own, but not everything is about direct-sales.
Not everyone wants a sustainable business. Some people just want to get acquired. This was even mentioned in a business class I had in the 90's.
Did you make money? Yes. Did your investors get their return? Yes. Sounds like a successful exit to me. Every week I'm seeing an article about Google/Apple/Microsoft buying a small company that had no hope of being profitable on their own. Everyone defines success on their own terms.
Getting acquired and becoming a cost center for another organization is not how I'd define success.
Yes, companies like this exist, but they typically are in the news because they're the exception to the rule.
For any sustainable business, the obvious part has to be how it makes money; the tough part is building it in a way that captures the customer.