Mumsnet lacks a quoting facility, so I will rely heavily on underlining. Also take into account that I'm not a skilled debator, and other people would be far more efficient in taking the OP apart.
Contentious topic
Bad sign. [Insert amusing sarcastic picture here]
DC often get an absolutely shit deal when marriages break up
Define "shit deal" and quantify "often". Otherwise this is unsubstantiated, ill-defined opinion.
and often have to accept their separated parents embarking on new relationships and having to be part of a 'blended' family with DCs from the stepmother/father added into the mix.
Your premise concerned the "mental impact of divorce" on children, and whether people severely underestimated its significance. You are now conflating the effects of blended families with that of divorce. Not all children of divorced parents will ever have step parents. Of those that do, not all will have step-siblings. These are separate issues.
Some step/parents that complain about finding these DC hard work and their behaviour difficult to deal with. Not exactly rocket science that DC will be disturbed by having to share living space, and time with their parents, with people they are not related to and not having the security of living with, and focused attention of, both parents which is their birthright.
You're still confusing step-parenting issues with issues of divorce, and you are implying that all children have "security" as a result of living with their parents, and "focused attention of both parents". That, unfortunately, is not always the case, and that, itself, may lead to the breakdown of a marriage.
A lot of marriages break up way too easily these days as the parents want to be 'happy' or have 'fallen out of love', 'want a newer model' etc, etc with the DC dragged along for the ride with no choice in the matter. Unsubstantiated opinion.
In terms of abusive relationships, it could be argued that you should know your partner and their background well before bringing children into the equation which seems to be a very controversial view
Absurd view. Are children of an abusive relationship less worthy to be brought up in an emotionally and physically safe environment, because the victim in the marriage didn't foresee the issues? Are they innately destined to witness domestic violence?
Or are you simply ignorant of the fact that living in a domestically abusive environment affects the children? Even if they are never hit themselves, there is a "mental impact". It's not a simple case of "well, if, the woman (because usually it is) does her duty and makes the best of it, the children will be fine".
Is it really impossible for adults to put their 'happiness' and new relationships, which would have an impact on their DC, on hold until their children are grown up?
Still conflating divorce with new relationships?
The damage is often played out when they become adults as children will accept almost anything from their parents as a survival mechanism until they are old enough to question it. It is thought that DC are resilient and will get over it but I think that is a fallacy self absorbed parents tell themselves to make themselves feel better.
And this only applies to children of divorced parents now? Not children of unhappily married parents, or children whose parents were in an abusive relationship, obviously. According to your thought process.
My 17 year old DD is the only person on her college course whose parents are still together. I find that shocking and really sad.
And this has what statistical validity?
AIBU in thinking that this is a ticking time bomb in an explosion of mental health issues in the next generation and the implications of divorce/separation on children's mental health should be much more ingrained in the morality of society?
Based on your post, YABU. No support for your assertions at all. Or internal coherency.
If you don't like my post, fair enough. Debate versus discussion is very much personal taste. The above is not MN style, which is why I don't use it here normally.