I think your point about judging others' lifestyles (for pottering about etc) is fair to a degree. That said lifestyles really do affect behaviours and beliefs to a greater or lesser extent. So it was also fair that poster said she recognised how miserable her parents were and really didn't want to end up that way herself.
I'm glad you have upbeat and happy friends of your age. But just because you don't recognise this with yourself or among those you know does not mean it's not the case for other people - as so many others on this thread have testified that their parents are this way...
Also you can't blame the viewpoints of the parents on the children. Nor their level of happiness or lack thereof. Everyone is ultimately responsible for their own happiness and attitude. But it doesn't mean these people haven't tried to help. The definition of insanity? Do the same thing over and over and expect a different result. You reach a point where you have no more energy to keep trying to change people who don't want to change.
And with regards to "disengaging", who honestly wants to sit and listen to misery on repeat every time you see someone? Or hear the same story on repeat 50 times, like a robot? The adult child's mental health is just as important as is the parents.
I don't think anyone on here has said they went No Contact, all of them are lamenting and finding ways around it from what I have read upthread. Whether that's limiting contact, deleting things off their dad's Instagram, trying to take them out or get them to volunteer. But I think by a certain age most people have an idea who they are and what they want to do or not do, and trying to get them engaged in something else is very difficult, unless they decided themselves to do it. It's a fact that the ageing brain's neural pathways are more ingrained.
This person even came on here for ideas about how to cope with it...
It might help you to also understand how hard it is for a child who grew up with kind, thoughtful and happy parents to see them change into a miserable husk, who refuse to do anything about it or even recognise it. That's a big part of the real tragedy here.
It's a grieving process, the people you knew and loved replaced by someone you no longer really recognise, their happiness gone, replaced with ranting about GB News and hatred towards people of a different skin colour, it's sad and upsetting and incredibly disorienting.