OP, I get you.
My friendship group is a bit different from mainstream culture in that lots of us are still friends with our exes. (We formed our friendships in the rave scene of the 90s, perfectly normal to be friends with your exes if they weren't dicks in that subculture).
Decades later, I see several of my friends still are friends with their exes, without it feeling weird, and where there are kids involved, it certainly can benefit the kids that everyone is grown up about it.
Not that our generation were doing anything new, particularly. I remember a couple of friends growing up, whose mum and step mum were friends, and my own parents have a close friend who was my dad's girlfriend for a while, years ago.
Mumsnet has this down as lack of boundaries. I think it's being a grown up about things and that not being ruled by jealousy is a good thing, although I appreciate not everyone would agree.
However, like any friendship, it has to be wanted on both sides for it to work. Your ex's GF is conforming to ideas in our society that are pretty mainstream, about how relationships should work, so I don't think you'll get anywhere with changing their minds.
Unfortunately, you do need to accept that they want a clean break now, and that it's time to let your ex do that, if that's what he wants. Allow yourself to be sad about it, as it IS sad that he is withdrawing in this way. For a lot of people that happens when you split so the mourning of that loss happens then. It's OK to feel sad that it's happening now. But you do need to accept it and let him go, just like when someone wants out of a relationship, it's their choice and there's nothing you can do, except protect your own feelings and your child's through this change.