I think a better, but still not perfect, way to define it would be “This person wants to do X, but can’t support him/her/itself doing it.”

Of course, if you are already rich it doesn’t matter and then it is a bad metric (one of the reasons it isn’t perfect.) However, I think it is a better way to define it. Someone writing a few books as a hobby and then stops are not a failed writer, but someone that wants to be a writer but just can’t support it is.

Basically I think the intent matters, but that is impossible to measure (and people lie about it). So being able to do it as a profession is an ok metric.