I mean ad revenue is definitely one model like this. But even Dropbox, Uber, Tesla, Netflix and many others would fit under that category.

Sometimes the generating revenue part itself is expensive to build, if you can't even get tons of people to use your product for free you're going to have trouble getting there (but that's a lot more for advertising-based revenue).

I think a lot fewer companies have no revenue stream that you're suggesting. Sometimes very early companies haven't figured out revenue, but they also haven't figured out product. Sometimes they're closely tied together, sometimes they're not.