I can only go off my own experience and the experiences of other people I know.
I had my first child despite being a young single mum and living at home with my parents. I wasn’t earning much and it was a struggle but I knew I wanted to be a mum and I made it work (admit I had family help in terms of living at home until I’d saved enough to move out).
I also know lots of people who in all honesty couldn’t really “afford” to have children but have done it anyway and things have worked out fine. I don’t know anyone who hasn’t had kids bc they don’t think they can afford it, the two couples I know who have actively chosen not to have them have made their decision based on not having maternal feelings and also having tried and nothings happened and they don’t want to go down the ivf route.
I really think a lot of this “can’t afford children” rhetoric comes from people having higher expectations of what they want their lifestyle to be like nowadays (probably largely fuelled by social media and FOMO). People in wars have children, families on benefits have children.
I honestly think people who really want them have them and those who aren’t really arsed one way or the other don’t but sometimes blame it on COL etc as it’s a convenient excuse to shut people up who ask.
When you really want children you’ll move heaven and earth to make it happen, hence why people go through rounds of painful invasive ivf to have them.
I think people have become more selfish in general (not saying there’s anything wrong with that) and prioritise their lifestyle/wanting to eat out/have nice cars etc instead of the expense of children. Everyone’s expectations have risen to what they consider to be a decent way of living and that’s bound to affect birth rates. I also think the internet/SM/phones etc play a part. People are so hooked up to tech nowadays they probably wouldn’t want to get of Instagram or whatever in their spare time to go rock a crying baby! People had more downtime to be filled before the advent of SM etc - boredom leads to wanting change/new experiences (and more sex!) = more babies!