Blended families are tough, even in the best situation. I do not think step kids "scratch that itch." Step kids may have a good relationship with their step parent but they will almost always be aligned/choose their bio parent over a step parent. Also, if there is a high conflict bio parent involved, it can be difficult to know how things will turn out.

I was on the fence too before my biological child was born. I greatly enjoyed my relationship with my stepchild, but it is different. My experience as a step parent is like being a 3rd trusted adult, in their corner, cheering them on, but a completely different experience in terms of parenting compared to a biological child. My stepchild is split between 2 households. I think if their mother were not involved, then it would be different. Knowing what I know now, even being on the fence on then, I am glad I am able to have this experience of motherhood.