Yes, there's lots of good men out there. But many women discover that the ones who are any improvement over the one she left, have little interest in a 40-something woman with kids.
The vow was "For better or worse, for richer or poorer, in sickness or in health, Til death do us part".
The vow was not "Til I feel unhappy".
You just never know about that. You’re right, it’s more difficult to meet someone if you are over 40 and have kids, but it happens.
I know a woman who had a severely retarded child that needed round the clock care. She met someone and they ended up married. The man was very financially secure and was able to provide excellent care for the poor child. So you never know.
For better or worse is the best way but sometimes staying in a marriage just isn’t an option if the welfare of the family is at stake.