I personally feel there is little point in picking over someone's ended marriage to analyze causes.
It's not abnormal for marriages to end. It's a very, very common and even normal thing to happen, especially after a major life change. Also, the main things that keep failing marriages together are: children, and inability to afford divorce. So a no-kids marriage where each partner can afford to move on? That dramatically raises the probability of divorce, not because it makes marriages worse, but because it makes it easier to leave a marriage that isn't working. And there are A LOT of people out there in marriages that aren't working who feel like they can't leave.
I've never read his blog, but from that post and this thread, I'm gleaning that as a couple they dealt with: infertility, the MAJOR lifestyle change of early retirement, feeling alienated from friends, a very serious and untreatable chronic health diagnosis, untreated mental health issues, communication challenges, a refusal to seek professional counselling, and infidelity. It's actually impressive that they were married as long as they were and sustained what sounds like a pretty decent relationship for most of it.
I would actually call that a success.
Cheating is a dick move no matter what the circumstances, but he sounds like he's in a much more compatible relationship now, so that's also a success.
IDK, maybe it's just my perspective as someone with a very similar diagnosis, who has been through a lot of similar shit, I didn't read that post with any sort of shock or horror. What I read was an account of someone who started their retirement in the wrong marriage and in the dark about his own health, which is an illness he would have had his entire life. He didn't get sick, he got properly diagnosed after having it forever. And now he's properly diagnosed, in a happier relationship, potentially going to be a father, getting proper mental health care, and neither his illness nor his divorce left him in any real financial trouble. He's voluntarily working to subsidize his spouse's retirement because he can. Love seems to be his main motivator to keep working.
What I read was a happy story where the transition part was hard, because major life transitions usually are hard.
But as I am fond of saying: an absence of difficulty is not what makes a good life. And what this man shared is that he has a good life.