Rent eats one of my bi-weekly paychecks entirely, leaving me just under a grand a month to pay utilities, car insurance, fuel (80 miles/day commuting), medical expenses (even after health insurance, those are substantial, at least for Americans), leaving little for food. But to even have that much after taxes, I need to put in a few hours of overtime at a job that pays >$17/hr.
If you have children, or any debt from having had a rough time getting to where I am now, $40k/yr won’t cut it. You have to have led a charmed life to survive on much less than what I make and still even be able to think about ever retiring.