I really don't believe the majority of couples go into marriage thinking 'oh well, I can always get a divorce'. I also believe that there are no guarantees. People change. And you cannot control it when that happens to the person you have chosen as a life partner.
I got married after we had been a couple for 5 years. We had our first child 3 years later. He was great - smart, funny, warm-hearted and generous. We both worked hard, shared goals, did our fair share of everything.
Then his dad died, and 3 years later his mum died. They weren't young, it was just mother nature. And his job changed until he hated it, but he wouldn't make the effort to find something else. He did find alcohol though.
He became sullen, withdrawn, lazy, constantly drinking, gave up on self care. I tried to be supportive, encouraged him to seek help and make changes - nada. I stayed for six and a half years. During hat time our DDs became teenagers and believe me, they noticed what was going on.
So in the end, I made him leave. The effect on our DDs? They have had counselling and support, and they are infinitely better than they were when he was still with us. They eat better, sleep well, their social lives have improved out of all recognition. They are confident, they are doing even better at school than they already were. And we are a relaxed, happy family of three. We have the odd squabble, but the dark shadow is gone.
I wonder how many people the OP is judging as 'should have plodded along in a less than optimal relationship; are living the same kind of life I lived for far too long. My only regret is not getting out years earlier. But you see, I was trying to stay for the sake of the children. 