Surely children are the only reason for staying in long term relationships?
I hope not! There's so, so much benefit for an individual and also for other people in their lives by they children or step-children or grandchildren or close siblings, in long-lasting relationships. Romance doesn't last long at all, but reliability and trust and respect are deep values which are worth a lot in my book. Especially when we're old and have been through things.
why would you think "there must be something wrong" with someone for not being able to have long-lasting relationships?
If a person wants a long-lasting relationship (or two or three in a long life), absent factors such as abuse, an inability to do the things necessary to get what you want would indicate the problem lies with that person. Either for consistently choosing the wrong people (people who don't want the same thing); or for not being able to do the necessary to get the thing. If a person doesn't seek long-term relationships in the first place, all is fine unless children are involved. I agree with you: children need stability.
Beyond a lifespan of about five to ten years I think very few couples would naturally stay together if it weren't for children or finances.
Personally, I find this is such a sad conclusion for an adult to have reached. Quite male, too!
Surely deciding something no longer serves you and allowing yourself to move forwards is healthier than doggedly soldiering on with something just to demonstrate you can have a long relationship?
I don't subscribe to these individualistic and transactional views. They aren't the path to fulfillment and deep happiness imo. I do many things that don't serve me out of duty and respect (such as raising children and helping my aging neighbours). Long relationships almost always have periods of dogged soldiering on: when your partner is sick, when a couple falls on hard times, when tragic bereavement befalls them. You don't stick around "just to demonstrate you can have a long relationship" - who cares? You stick around out of love, duty, honour, self-respect and respect for your partner. Those are all good and decent values in my book. Far, far more meaningful and rewarding than self-serving attitudes which only result in loneliness, ultimately.