this is too situation-based to answer, really. they do what they want or need to do. maybe that's start another, or maybe its work at a more normal job.
It would be great if (ex)founders could share a little bit of their situation too, for context.
well, i'm currently working on something that i do after my 9-5. if it goes under, its not much of a loss -- i'll probably do another. however, if i was working on it AS my 9-5 and it went other, i imagine i could be in a bad financial situation and need to get a solid job.