I think that just as DSC and their parents need to accept that their nuclear family is no more and the step family cannot operate entirely as a nuclear family because it's not one, step parents need to accept that finances also cannot operate as a nuclear family either. It's more complicated than that.
I struggle with this because whilst I see what you're saying, I think all too often the onus is put on the step parent to accept X Y and Z when it should actually be the parent of all the children who is expected to do X Y and Z. Why should you accept that your child's other parent likely won't pay a fair share toward your children because they have other children, simply because you chose to marry them?
It really bugs me that so often the onus isn't put on the parent to all of the kids in these situations to be the one to think ahead about this.
My husband has two older children, I am aware that means he is stretched more thinly than I am as I only have one child and I may, or may not, choose to then pay more toward some things for our child but I would be royally pissed if it was an expectation. Damn right if our son needs something, or its Christmas, or his birthday, or a holiday, I expect my husband to at last offer to pay toward our son, as his child, just like his older children's mother expects the same. Why should he not? If he wasn't able to do that, why is the onus not on HIM to reconsider having another child with me?
I don't think it's right that step parents should accept having to shoulder the majority of the financial burden of joint children simply because they chose to marry someone with older children. In the same breath, no one would expect the older children's mother to shoulder most of the burden simply because their ex has more children with someone else. I am aware a step parent chose to enter into a relationship and have more children which the ex did not, but I don't think it matters. I am also aware CM can reduce in the event a parent goes on to have more children with a new partner but I've rarely seen anyone on here agree that should be the case. Most people seem to think (rightly) that their contribution to children should remain the same. I don't think that expectation should be any different for new joint children either.
Men should be paying their fair share for any children they choose to create, whether that's with an ex or a current spouse. And if they don't, that is unfair and not something you should just accept and expect and the blame should lie squarely with them for choosing to have more children than they can reasonably afford to contribute equally toward, not the step parent for marrying them.