Blanket statements like GP made are not true for everybody.
If I (and my partner) had the opportunity to work less during weekdays and more in the weekends I'm still not sure we would do it. There is something mental about it for me at least.
Besides she's a nurse, so she works every third weekend anyway. In that case weekends become really important for me.
I understand your point, though, I work max 30-35 hours a week, often on weekends, and spend as much time as I can with my son and wife.
I'd adjust that to "If you and your family and most of your friends are able to be more flexible". I went back to a mostly conventional schedule after I realised that while I was doing a lot that I wanted to on my own, spending more time with friends was a higher priority. I can see that if most of your friends are academics or similar then it could work better (indeed I found that in this remote work era it's become easier to meet people during "work hours", so if that sticks then maybe it'll be time to reevaluate).
I guess I would be fine if I was trading those 4 hours of freetime I normally would get on Saturday for 4 hours on a weekday. But that doesn't seem to be what's being suggested.
Also, given the nature of how our days are organized in society, there tend to be events that happen on the weekends that do not happen on week days to accommodate people's work schedules - concerts in the park, community events, social events, etc. so moving some of your work to the weekends will mean you could miss out on those.
EDIT: I can understand 'kids' aspect. Outside of that, if I'm doing the things I want to do at the times I want to do them, the notion of 'weekend=do_what_i_want' isn't as compelling. I try to schedule in things I want to do when I want, weekends be damned.