You might be happy as a PhD student, but your housing and life is entirely dependent on the university. You are one academic violation away from that all being in jeopardy.
What happens if you get a spouse and child? Will your current housing and savings be enough to support them and yourself if you both decide you want to stay with the baby instead of working?
What if your government passes legislation that costs you your job, your housing, or your healthcare?
The $10m money is about not worrying about anything other than global economic collapse and/or massive wars.
"Not worrying about money again" means never again. Not just while you manage to hold down a job.