They might if they new mother had met her new baby in many occasions before she decided to have it! The scenarios you list are just nothing like a woman who decided to marry a man with existing children
@WhateverJudy
Your defence of "you knew what you were getting into?" is naive in the extreme.
It take no consideration of how people, relationships and overall family dynamics can change over time.
Your point is akin to berating a woman who'd married a man after dating for 5 years only to find after a year of marriage he's having an affair. Would you say "you knew what you were getting in to when you married him?" or would you offer kindness and sympathy?
I've been "lucky" as a SM in that DH and his Ex have largely worked together to co-parent and he and I are in sync about "how things work" in our home. We have house rules/age appropriate chores and clear consequences for poor behaviour that either of us enforce regardless of the child involved.
Though it isn't really luck is it? Assuming people will behave decently is surely not an unreasonable expectation?
My friend is not so "lucky".
She dated her now DH for 3 years. Took things slow. Did all the "right" things.
In that time her (now) DH and his Ex had a cordial relationship and were effective co-parents (after she left him for OM who she moved in with). All was working well for all involved.
Then they got married, she got pregnant and all hell let loose.
Ex (jealous that the husband she'd kicked into touch was getting on with his life whilst her OM had turned out to be a good looking but feckless fucker who had no intention of marrying her never mind having another child) turned from a reasonable co-parent to the Ex from hell.
Contact constantly disrupted and weaponised.
Friend being slagged off to the child and told she couldn't tell him what to do who then played up as a result.
Constant phone calls/texts to friends DH about what she was doing wrong, could do/couldn't do (often contradictory).
The list goes on...and it's put a huge pressure on her relationship. Her DH is understandably shit scared about not seeing his child but as a result allows himself to be manipulated an bullied by his Ex to his wife's and their child's detriment. Frankly I don't know how much longer it will last.
Did she know what she was getting into?
For my part as lucky as I have been there have still been testing times as my step child has grown up and we've all had to re-orientate ourselves to changing needs, or simply plan around the impact of Ex moving 20 miles away (the burden of which fell on me and DH in terms of travel as usually reasonable Ex decided she wasn't going to do her share of hour long round trips despite being the instigator of them).
This attitude of yours (and others) is pretty much exclusive to SM's. I rarely see it dished out to SF's nor in any other circumstance such as my 3rd paragraph example.
I don't ever see as much much ill disguised malice, glee and superiority on any other part of MN than this topic attracts, nor less empathy, support and understanding.
My GM had a good name for people like this. Candle snuffers. In other words people who get pleasure in blowing out your light to make theirs seem brighter by comparison.