Unemployable is not wholly in your hands because burnout is not at all in your hands, as evidenced by the startup culture that expects and rewards long hours even though they know how unproductive and unsustainable that is. Burn out is just another way of saying "major depressive disorder" or some other mental illness. It is not glorious and it's only in your control if you never start down that road.

Is it possible to create a startup and working normal hours while still having tight deadlines? Sure. Accountants do it for startups all the time. Why can't founders or developers? Likely because they're spending too much time working long and late, getting done in 4 hours at 2am what they could have accomplished in half an hour at 2pm. It is possible, but I have yet to see it happen.