The responsibility for his children sits squarely on is shoulders. Don't collude with him in scapegoating his NP.
I would say that in theory, this is absolutely correct, in practice not always. I think in some case, sm can put a lot of emotional pressure on the nrp who feels cornered between his children from a first marriage and his new partner.
My OH has a friend who is finding himself in exactly that situation and is totally lost as to what to do (he's asking my OH as it's the only man he knows with scs, but I don't think my OH is best to talk to about this!). He was single for 5 years after separating amicably with his ex wife. They remained on good terms and he saw the children every week-end. He was a very involved, flexible dad and it was all going well for his ex, him and the kids.
He then met his new partner and was over the moon as he had been starting to suffer from loneliness and desperately wanted that family life again. At first everything was great. She was fun, pretty, independent, and a great mum to her two DDs. They decided to wait for them to meet the kids but by the time they did, they were already very much in love. It didn't go as well as they'd hope as their children are very different and it became clear they had different views on how to raise children. They chose to ignore it though and pretended all was fine and convince themselves all would just get better with time. He ignore the constant little comments she made about his kids, the fact that she used to get grumpy when he went to parents evenings with his ex and things like that.
They'd talked about getting married and maybe having a kid together, but he wanted to take things slowly until she announced that she was pregnant. He was taken aback as she was supposedly on the pill, but still very much in love with her, he was over the moon. Things were ok until the baby was born and all of a sudden, she decided that she wasn't happy with his kids being there every week-end as she needed time with him and the baby alone and that it would work better if it was every other week-ends when her kids were not with their dad, that he shouldn't be going to parents evenings with his ex any longer but arrange a separate time to go on his own or with her, and that now they had a baby and she had decided to become a SAHM, he would need to cut down maintenance but also the special treats he gave his kids.
The baby is now 8 months old and my OH friend is very very unhappy. He feels emotionally blackmailed to push his first kids away and he is exhausted trying to argue that he doesn't want to do that. He has been considering leaving her, but he is devastated at the prospect as first he still loves her, and second he feels this would be a massive step back with yet another ex and child, and still not the family he wants.
I personally think that he will go along with her demands as ultimately, this is the easiest solution and hope that his kids will accept it and not feel it as a rejection. My OH has told him to be careful as they are entering teenage years and told him about my DS and his relationship with his dad. I feel for the guy. I feel that his new partner has manipulated him from the start, but I'm probably biased. I still think that she should have made her demands clear before deciding to have a baby with him.
So yes, I do think the onus is not the nrp to decide what to do for best, I think that some just feel that they are doomed if they do and doomed if they don't.