First of all, I don't think marriage is imploding for everyone in this country. It's become very much a sign of privilege - the most affluent in the UK are the ones who are most likely to marry.
So in my relatively affluent circle, 99% of my friends' parents are married and have all been married for 30+ years, a third of my friends are married or are engaged. I am getting married myself in 6 months' time (I'm 28).
Secondly there is some research to show that this is solidifying inequality. Married couples live longer, are more likely to stay together, are happier and have children who are more likely to achieve, compared to similarly educated & wealthy non married couples. This due to a few things, one being the effect of shared finances making the family more stable, the other something a bit more mysterious - the psychological effects of making a lifelong commitment in front of everyone, perhaps.
You talked about a high rate of marriage breakdown. In recent years the number of divorces is down. And for people who marry between the ages of 25-35 for the first time, it's very likely they will stay together (I think it's something like 70%...but I may be misremembering this).
So marriage is definitely worth doing and RA sounds destabilizing and a thoroughly unpleasant way to conduct a relationship, involving absolutely none of the compromise and personal development that is one of the most rewarding things about long term relationships.