But aren't planned/wanted kids intrinsically more unlikely to be born into families that are able to support them financially and emotionally?

People who decide to get kids will always be more likely to do so when they can actually support them.

People who get kids by accident (or even against their will) will be more likely to put kids into unstable environments.

Kids in unstable environments are more likely to grow into adults that do crime.

One way of avoiding these kids is abortions. But you could also provide stable environments for kids that are actually born (which is very much what we in Europe did). Allowing abortions won't change a lot in a society that already does what it can to support all children and their parents — it however will change a lot in societies without social security.