You really don’t sound great, OP.
I would be completely horrified if my mom ever thought (and worse, could publicly state) that she could realistically « step on children » (worse, kids she knows and partially raised) to save me, with no problem whatsoever.
Irrelevant of me being her daughter and her genes, it’s so messed up to even be able to picture such scenario in your head, and (I know you weren’t the author of that original post) think « yep, I can relate and would totally do that. » I couldn’t personally imagine stepping on a kid, full stop. Death or life situation or not and you are out here, publicly happily agreeing to scenarios in which you would cause fatal harm to your step kids? Step kids you say you get on fine with? That’s horrible, and I would seriously second guess having had a kid with you if I was your partner.
Now to get back to the topic. I am not a step-parent, but have a step-dad who I am actually closer to than my own mother/parents. He did have to deal with issues with my dad, and even my sibling as he grew up, but I think he would vehemently disagree with your vision of things tbh. My mother has since remarried with a different man with whom she has had another kid and this man has had no interest integrating in the family and personally I will never understand why people who don’t care to integrate an already former family, make the decision to become step-parents when they hate every single minute of it. We keep up the good face for my little sister who is just 3 but it makes me very sad for her because she will actually never experience the childhood we had in a family that’s truly United and love each other and where everyone has its place.
Personally I wouldn’t mind being a step parent but would be extremely cautious about bringing a step parent in due to too many step parents wanting the relationship with their partner but not actually wanting the kids in tow, and the dynamic it creates when an extra kid from the new relationship is then added (where tolerance for the step kid usually further decrease).
I personally find it so sad that too many step parents make kids feel unwelcome and unwanted in their own homes. It might not be your case, OP. But the fact that most of what you hate about being a step parent has nothing to do with the kids themselves and all to do with your partner (that you chose!) and his ex. And yet, you hold them responsible (vs your partner) for your unhappiness, is quite sad.