There are plenty of step parents who almost never see their partner’s child

Yes. And those people would be assholes if they threw a fit over “not signing up for this” if their SO ended up having to take primary custody. That’s always a possibility as long as they have parental rights and as long as the children are alive. What if the other parent dies? What if they have to move away? What if they have to go on a six month long business trip?

Consider that a lot of people in poorer communities have children with multiple partners

Quit making this a poor people thing because it’s not. And yes, those people are still expected to exist as a blended family otherwise they’re assholes.