Very interesting discussion. I worked 4 days a week from the word go as I was the main breadwinner. So I went back to work when ds was 7 months old, he went to nursery and I paid for a full-time place. I switched jobs when he was 2, he also changed nursery and stayed there until he went to school and worked full-time, but was able to work from home one day a week in that job. I got made redundant from that job after a year and worked for a law firm FT. That was when it got difficult because I had to be in the office every day and I just couldn't do the hours required.
So I changed jobs again and went into a well paid professional role, but it was definitely more of the mummy track. I was there for 4 years, working FT and then I had a year out working in a very badly paid role locally, but that showed that we could manage on my earning a lower salary and that working part-time was the way forward.
So now I am back in a well paid professional role. I work 4 days spread over 5, so effectively school hours, and I work from home most days.
It is the mummy track - but I'm not sure I would have made it in my profession even if I had not had kids, or a stay at home husband.
I think there are a lot more options available now, I know people with and without kids who do contract work - so you might have a 6 month contract full-time but then you can take a break between contracts eg for the summer holidays. I work at home. It is easier to fit work in - so to the person who feels demoralised in their current job and is one bad maternity leave from giving up, I wonder if you should look for a different role or even different career before you give up? I'm sure a solution exists for you.
As for the OP, I don't think it's unreasonable for the sole earner to wonder when/if they can stop being the sole earner.