This is zero-sum thinking. App.net doesn't have to be as big as Twitter to be successful. There are many pay services to compare it to. Metafilter is one.
You're right it wouldn't need to grow to the size of Twitter to be successful, but I don't see how it would grow beyond the HN set (and just a subset then). I can't think a single reason anyone would pay for Twitter, when free Twitter already exists. The only pain point App.net really covers is for 3rd party devs who want to quit their day jobs to sell Twitter iPhone apps but can't any more.
You're ignoring the possibility that it will be different from / better than Twitter.
Yup, they don't have to be big. Then what's the point of developers to develop on top of app.net API while their user base is small? Less revenue, more "freedom" to develop?