Agree with you flump, that maddy missed an important reason for co. Or rather, she referred to it when she mocked the idea of gps having to "respect" their ac's and cil's "boundaries." Imo, if a gp who refuses to accept their ac's/cil's boundaries regarding the ac/cil's home/kids/personal lives, it's the gp who is being "controlling," not the ac/cil. Maddy, why shouldn't gps have to observe these kinds of boundaries just like anyone else? Does being a gp give one a special pass to step on ac/cil's feelings and ignore their wishes? Is it somehow ok to disrespect parental rules, even in front of their children, just because one is a gp?
I don't favor cos, but I would definitely lower contact with anyone who broke my rules for dd when she was a child or who openly disrespected my authority as her mum. Responsibility? Don't gps have a responsibility to treat other with respect? Yes, even those who are younger than they? Don't they have a responsibility to show their gc they respect the gc's parents (unless the parents are abusive, of course) and to avoid undermining the parents? Also, imo, parents have a responsibility to protect their kids - and yes, themselves - from anyone who would disrupt the parent/child relationship the way undermining gps often do (again, different if there's abuse involved).
I feel deeply for any egp. But as flump points out, it's not always the ac or cil's fault.