> it's like a time limited contract

Except it is not. You are probably very well aware that maternity leave in Germany protects the mother's job up to three years. There is never certainty about when the mother will come back.

that's irrelevant, because there is a certainty that she will come back, and the contract for the replacement should be worded accordingly.

What I am saying is one can not put someone else's career growth on the line over this.

what is the alternative? force the employer to hire both? how is that reasonable? this is no different than getting contracted to work on a project, when i don't know when the project will end. i know the end will come eventually, and i can prepare accordingly. as i said before, this is a conflict between the needs of the mother and the needs of the replacement. protecting the mother (or the father, since they can take time off as a parent too) is simply more important than protecting the replacement who knows from the start that this will be temporary. they can plan their career accordingly.

Couldn't it be that we have been optimizing for economic output instead of optimizing for a system that keeps a nuclear family living well enough on a single income?

that is true, and i agree with you that this is a problem. but the solution is elsewhere. part of the problem is that raising children is still not supported enough. and not working is stigmatized. my dad was in this situation. as a single parent he could not work and had to rely on state support with zero income. once we got older he had to switch careers because he could not work in his old job due to health problems. that's basically the situation that any single mother faces who did not work and thus had no qualifications for any decently paying job.

and that's the problem right there. by only allowing one person in a family to work, we are greatly disadvantaging the other person. and that is the reason why living on a single income will never be popular. instead we need to address this problem in other ways. one way would be to reduce everyones (not just parents) work hours to 20-25 hours per week, so that parents can split the time raising children evenly and both parents have an equal chance of having a career at the same time.
the other idea would be to provide stay-at-home parents with resources to earn qualifications that they can use to work once the kids are old enough.

what we can't do is to expect stay-at-home parents to remain without qualifications for the rest of their lives. that would create a large segment of unemployable people, leaving them dependent on the state, or on their divorced earning partners, (which has the secondary effect of making divorce much more difficult because the earning partner would have to continue to provide for their ex, which means they possibly can't marry again)

especially among women this would create two classes of people. those that have a career and those that don't. you ask why the fertility rates are going down? this is one reason.

getting rid of weekends. Isn't that further propagating the idea that pushing our society to more work is a good thing?

we are not getting rid of weekends. we are adding flexibility. the maximum time to work in a week doesn't change, nor the right to a day of rest after working 6 days. (not sure if such a right exists, but i think it should.) added flexibility would also be beneficial for those who are not christians. the jewish day of rest is saturday. in islam traditionally people take a day off work on fridays. locking the day of rest to sunday is a very christian centric practice, and not compatible with a multicultural society.

also, in times where companies experiment with 4day work weeks i see absolutely no risk that allowing work on sunday will lead to people working more days. and making a day of rest mandatory is a simple protection against any abuse of that. allowing shops to open longer will also create more jobs. the only ones at risk are those who are seelfemployed. but they are already at risk. noone stops me from working on sundays, only my health, and my family.