To be a professional anything means that you are in a business, like it or loathe it. If you can't pay the bills you leave/go bust. If you can't stand the lifestyle you leave. If you realise you can earn more money for less hassle and get a mortgage etc you leave ( or become semi-pro). If you realise that you can't earn as a pro what you currently earn as a semi plus your day job you don't bother becominf pro.

Or you may just grow out of it sit back and enjoy the music/performing without the pressure of thinking "I need 4 more gigs that month and why doesn't x return my calls and how do I pay that bill and s**t is that the bailiffs".

A lot of the longstanding "professional folksingers" listed above, also make a portion of their income from non-singing activities, instructional DVDs (Kirkpatrick), instrument teaching, school's work (eg Pete Coe), web design (Dick Gaughan) etc.

It also helps if you have a royalty stream (MacTell) or at least have some money from your back catalogue.

Steve