Of course it's healthy for both individuals in a marriage to get out of that marriage they're unhappy in. Is this really up for debate, considering the wealth of examples?
So divorce is the healthy way to end a chronically unhappy marriage, yes. But surely we would agree that a happy marriage is healthier. This is a bit like saying that a pneumonectomy is a healthy end to one’s relationship with one’s lung.
If two people are unhappy with each other, then they should do whatever they want -- stay, go, who cares?
But parents aren't just two people. Life outcomes for the children of divorced parents are significantly worse than for children whose parents remain together.
The first question here is philosophical. I disagree with you as strongly as I could possibly disagree with you about anything. But it's ultimately philosophy and there's nothing much else I can say about it. (Well, beyond the obvious arithmetic, I guess. They are quite literally not just two people any longer.)
But as for the life outcomes of divorced children? That's quantifiable and the data simply disagree with you.
There is no shortage of screwed up kids from continuously married parents (or stable kids from divorced families). Divorce is like measuring the effect of radiation, rather than the cause. The issues that lead to divorce are the problems that mostly screw up kids, not just the divorce itself.
"The Consequences of Divorce for Adults and Children" (Amato & Irving, 2005), even has in it's introduction:
"Available research suggests that these associations are partly spurious (due to selection effects) and partly due to the stress associated with marital disruption."
Of course everything you've said is true. Nobody critical of divorce is saying, "it's the literal divorce that's bad for kids and as long as you just stay together and change none of the behaviors that lead to divorce, then that's better than getting a divorce."
But on the other hand, upper-middle-class Hacker Newsers who got divorced because they were "unfulfilled" probably shouldn't point to extreme cases of abuse and neglect to justify their decision, either.
All things being equal, people in this latter category should try to work it out. (Nobody disputes that there are extreme environments for which divorce is the only option.)
Finally, as always, we are talking about averages. Sociology doesn't have a proof-by-counterexample. If I say, "poverty leads to worse outcomes for kids" it's not a legitimate response to say, "there are some wealthy kids who do bad and some poor kids who do well, so you're wrong."
[0] argues that there is a differential impact of divorce on the children depending on the type of family they come from. You can divide families into two broad groups – high-functioning and low-functioning. High-functioning families, the parents may be unhappy, but they try hard to hide their unhappiness from their children, and the children may have no idea the parents have any marital issues. Low-functioning families, the children are very-well aware of their parents' unhappiness, they witness constant fighting, even abuse and domestic violence. Children from low-functioning families often experience their parents' divorce as a relief, and can even benefit from it. Children from high-functioning families, their experience of their parents' divorce is often much more negative.
