The problem is that unless you always save an increasing proportion of your salary from the moment you start working, most people are stuffed if you want to make a drastic lifestyle change like one person stopping working. You build up your lifestyle as your salary increases from first job income levels, a few years' pay rises, things start to feel a bit more comfortable, you meet someone and your two salaries continue to increase, you buy a house and a car maybe two, the mortgage may be cheaper than your rent, no kids yet so you get a nice TV or two, gadgets for the kitchen, nice furniture, you go out for meals, nice holidays etc...
Kids come along and so you have to cut back but everyone's got a level they don't want to go below.
If you suddenly cut a salary out of that you almost have to go right back to how you were living when you were poor and single. I mean we're not talking one less holiday and buy supermarket brand instead of McVities. You can't make a whole salary up out of that. We're probably talking one UK holiday a year, in the autumn or spring becuase summer is to expensive, or camping (but you can't afford the equipment), one old car that you need to keep going for 10 years, second hand clothes and getting suits mended when they rip in the crotch (I'm talking to you DH!!!!), going round several shops for your weekly shop instead of doing it all at one because it's easier, no pay TV, nothing in the way of hobbies that cost money...
This is what I'm trying to get across to my DH at the moment. We aren't cutting out a salary but I'm expecting twins and our childcare will be almost as much as my part time salary between when they are aged 1-3. He thinks we can manage with just a little belt tightening but I know we are going to need a lifestyle overhaul. TBH I know we won't do this, we will tighten our belts and some things will naturally fall by the wayside (no more international holidays with 3 preschoolers...) but we will spend on interest free (hoepfully) credit cards and then pay it all back over the next months and years once the 30 free hours childcare kicks in for them.
FWIW DH earns 65k, I earn 20k from my 3 day a week job and 6k from a sideline (all that is gross), and we have a lodger paying 600 a month. At the moment we are spending all our salary income each month which is shocking, although the lodger's rent is usually all saved. I will need to continue my job while the kids are pre-school for my own sanity but I'm hoping that once they are all at school I can grow my sideline and make enough from that to quit my day job.