It might be worth considering a mixed or hybrid path -- if you can get a high-paying traditional (software, finance, etc.) job, while being very frugal and saving money (and you don't have much or any debt), you can get pretty quickly to the point of having a very comfortable savings buffer. If at that point you use your savings for living frugally, but not for funding your startups (do that with Other People's Money), I think you'll have more runway and more options than you might otherwise, and should you find that the startup life is not working out you should have a good backup trajectory available where you go back to a high-paying traditional career and retire.

I'm curious why you think (4) seems unlikely -- is this a fact about your personality, or about the world (i.e. you think there will be no normal-ish jobs, and/or the world will not survive to see your retirement?) Considering again high-paying jobs like software and finance, I think the math shows that a very frugal person can effectively retire after very few years in such a job -- perhaps as little as a decade, certainly as little as two, which is similar to your stated startup timeframe -- depending on what your goals are. And once you're in "effective retirement" (i.e. you don't have a problem supporting yourself), then you're in a great position to try to get a startup going.