Like everything else on Mumsnet, I think it’s all dependent on circumstances. Taking just the financial situation into consideration:
If you’re a low earner who is partnered to a low earner, then it might seem more straightforward to leave as state support will kick in to support you and your children.
If you are a medium to high earner then the situation is similar, as you can make a lifestyle adjustment but will be ok.
If you are a low earner partnered to a high earner then not only do you face the largest adjustment but it is quite likely that your STBexH will use their substantial resources to make your life very difficult during the process.
It is not always possible to ‘earn more’ as many sectors are badly paid and, once you get past a certain age, your chances of switching to a higher paying job begin to dwindle.
In many parts of the country there is very little availability of rented accommodation.
Even ‘going back to your parents’ is lucky in some ways as many women do not have living or local parents.
Posters like to surge onto relationship threads and say ‘money or lifestyle doesn’t matter’ but I think it must do, or why does everyone ordinarily work towards financial security for themselves and their children? A decision to leave a long term relationship is fundamentally a huge blow to financial security, as generally all assets then need to be split into two.
It’s a difficult situation and I do understand why many women may choose to wait until their children are fully grown before leaving.
Edited to add: I forgot to say that I don’t think MN posters often take into account the impact at different ages. Becoming single in your thirties (when you are young, pretty and the chances of finding a new partner are high) is likely to be quite a different prospect to becoming single in your later forties or fifties. The dating threads are eye-opening to read!