Irreplaceable, eh?
Golly, you're so much better than me since you're NATURAL!
Yes. The graces that come naturally with natural parenthood come no other way. It seems so obvious that it shouldn't require proof. But if you want empirical data, there's tons of it out there.
If you want biblical evidence, here it is, in Jesus' rhetorical question:
Luke 11:11Golly, you're so much better than me since you're NATURAL!"Which of you fathers, if your son asks for a fish, will give him a snake instead?"
The graces that come naturally with parenthood are not the only graces dispensed by God (!). Clearly, grace is working through anyone who adopts a child. This is why adoptive parents can be great parents --even better than natural parents.
But adoptive parents never get the same graces that come through natural parenthood. And this is why, statistically, stepfathers and live-in boyfriends tend to abuse children at a higher rate than the children's natural parents.