I have successfully worked relatively senior positions (principal software engineer at a couple of startups) working less than full time. So I don't think that's necessarily the case, especially if you can build the skills to be very productive.

It's definitely harder, since less companies will hire you. But on the flip side, since you've already negotiated shorter workweek it's easier to get that at other companies, since you have proof you work well with less hours.

I talk about both the skills and the negotiating tactics involved in my book (https://codewithoutrules.com/saneworkweek/).