The bold part is interesting. Chances are high, so you don't mind if the state just assumes the chances were 100%. Why, precisely, do you assume the grandparents would want the product of their child's one night stand (potentially) or failed relationship (by definition) to reap the rewards of their life's savings?
The purple part is also interesting. The adopting parent is going to be punished because they decided to be a part of their step-child's life, but the relationship didn't work out with the biological parent? That seems unusually cruel. The second part, where you talk about dismissing the beginning scenario, intrigues me. Let's take a couple, and for the sake of statistics we'll make it a "single mom" and two men. Let's even give her the benefit of the doubt, and say that she was in a bad marriage that ended when the child was still an infant. The father is on the hook for child support. Now, the kid is five or so, and she finds the love of her life. When the kid is seven, this new guy even officially adopts the child. Unfortunately, when the child is ten, things turn south and there's another divorce.
So. The person who is responsible for the child coming into being should pay child support? Or the person who adopted the child should pay child support? And why does the mom in that scenario, who's on the hook for various bad choices, simply get the child-rearing paid for?
What if, in that same scenario, she'd simply palmed off the kid when it was first born, giving it up for adoption? Should she have to pay for part of the child's upbringing? If we're to follow the logic you have employed throughout this thread, then a mother giving her baby up for adoption should be on the hook for some financial obligation. No? Why not? A mother can terminate her maternal rights, give up a child, and be under no financial obligation. A father cannot simply say "no, I don't want to be a daddy, and I don't want to pay."
Of course, that to you is a rant, and it seems perfectly fair, and I'm the one being unreasonable. Think that all you'd like, but it won't make the system any more twisted than it already is. The laws are crooked and broad, and generally unfair. I'm glad you see a non-custodial parent as responsible, financially, for a child they have little or no say in raising, and the custodial parent as automatically responsible and worthy of being paid for their services. I've heard the world is much easier in black & white

I'll still stick to the notion that cases should be heard on their own merits, and that if the child is what is important, the system should not be skewed so that it's far more about punishment than about providing a safe, healthy environment for the child to grow up in.