What happens, longer-term, to the founders that fail? There's many forms of failure, according to PG [1] it's simply a matter of "keep going, don't get demoralized", but from my limited perspective (I've never started a startup) it's more complicated than that, you can also literally run out of money (either personally, or as a company not getting more investment),
or you can rationally decide to stop because the value proposition just isn't there in the market (noone needs it, or they need it but aren't willing to pay enough for it to sustain it). So, my conclusion is that startups sometimes fail for "good" reasons.
What happens to founders afterwards? Do they start another startup? Do they continue in a "job" (career)? Do they start from scratch or move immediately into a management position? Are investors willing to invest into failed founders? Are companies willing to hire them? Do they value experience (similarly to an MBA, or similarly to an employee, or not at all)? Maybe they get hired by VCs to either mentor other startups, or help decide if they're worth investing in?
The opportunity cost of starting a startup instead of continuing in your career can be significant (5+ years "lost"). I imagine Y Combinator and other accelerators/VCs have some relevant experience with their alumni to answer this question. Is there any data available, an article or similar?
[1] http://www.paulgraham.com/die.html
One thing many people forget about startups is that you're building assets, not revenue. The company, the code, the whole customer acquisition process, customer loyalty... all that is worth something to someone and can be sold at a price. We were acquired by a startup that just wanted to grow faster. It was another sales channel for them and far cheaper/faster than putting marketing dollars.
So there's rarely ever truly a failure. Just waiting for the right buyer if you've built something decent. This is usually what they mean by "don't die". I found that it's easier to sell a company than get investors, though.
After that,
1. A VC who didn't invest gave me several fun jobs teaching programming (his core background was education). I've been doing it on and off for 3 years now, training for programming certifications, and it's been a good portion of my income and connections.
2. Joined a couple of startups as a CTO, all fellow founders who I met in accelerators. They didn't go well and maybe I was not competent enough for them. Maybe they just had limited resources.
3. Joined a Big Corp. Friends thought it was not part of my personality to do so. But really, it was trying something I hadn't done, and proving to a relative that I could. It seemed like a step down, so I stopped after half a year.
4. I regularly get and turn down CTO offers, sometimes two a month. Many were more established and well funded, but I'm not willing to commit to them.
5. I went back to freelancing. I make about 5x more than I did before my startup. I specialize in MVPs, so my startup experience is essential to current income.
But notably, I didn't have a lot of options when starting a startup. Had I gone the non-startup route, I'd be working for a company in some large team, making $1000/month in a developing country.