This is a peculiar case all round, but I think it's an important principle to tie maintenance payments to biological fatherhood, not 'acted as father' role.
Otherwise every man who fathers a child but has no interest in raising it will say they don't have to pay maintenance. Or who walks out on a child later in life will say they are no longer the father figure so won't be paying maintenance. You can't have it both ways round. If you want to be able to hold biological fathers to their financial responsibilities (as we should) we cannot also say it ties in to playing the father role or not. The two things are rightly kept separate.
A moral/ethical requirement to keep maintaining a child you have helped raise is perhaps there in some cases. Trying to make this law, is very difficult. What if a child has a bio father and a role-father. Do they both then pay maintenance?
I had a step mother role to two boys till a couple of years ago. Should I be paying maintenance for them now I have no relationship with their father, no contact with them now, and they have a mother and father already? This isn't all that dissimilar to what is being proposed by some on here.
My DPs son has a child with a young woman who already had a child. They have split up. He is the older child's father figure and continues access with both of them, and treats them the same, buys them the same. However...he pays maintenance for his child, and the bio father pays maintenance for the older one. Should the bio father be allowed to stop this and have it become just DPs son's responsibility? Should bio fathers be allowed to shirk all responsibility?
If you remove the tie between biology and maintenance, it becomes a minefield.
I'm sure the father in this case loves the child just as much as ever. But the maintenance was always the bio father's responsibility and he and the wife conned the father.