It depends a lot on the circumstances of the individual and their lab/supervisor. If the environment is supportive and a sufficient stipend is provided, then it

It depends a lot on the circumstances of the individual and their lab/supervisor. If the environment is supportive and a sufficient stipend is provided, then it's achievable without rich parents. The environment is very difficult to evaluate from outside, but the funding is an important go/no-go gate: if you can't persuade someone to fund you as a PhD student, you're not in a great position for the post-PhD job crunch, and the struggle of trying to fund your studies in the meantime makes a difficult situation worse. You're right, though - having a fall back is very useful. I had eight years of outside work before starting mine, which meant that my "in case of disaster" plan was simply to go back to my old career (albeit with lost time and earning potential).