I think that for a lot of entrepreneurs there is probably never a point where they truly give up (and this is probably a bad thing, not everyone is made for it). To me part of being an entrepreneur is a bit like being an inventor, a tinkerer - there is always something you want to fine tune or an idea worth exploring. I can easily see myself following that path until old age, without necessarily ever making amazing money.

Saying that, most careers are dead. Obviously in some areas there will always be a direct path of experience where a true career will exist, but the days where you went from junior to intermediate to senior to manager to vp etc are dying fast, especially with companies people who frequent HN are likely to want to work with. Do you want to take a job where they say you didn't do two years as senior developer or marketing assistant etc so you can't have the job?

More and more companies are starting to recognize the experience gained in the field and it is easier than ever to move between fields and jump "stages". Not all companies are like that yet, in fact maybe most aren't yet, but as they lose good candidates to companies that are then they will change.

So I wouldn't worry about what happens if everything fails, if you're willing to try the life of an entrepreneur then survival post that is going to be easy.