Doesn't matter which situation is closer, they're both nothing like a marriage. There are more than 2 ways to think about it. If marriages are like a car, then there's also the option to ditch it and not get a new one. If it's like a critical body part, someone is way too dependent. Also, those relationship advice books focus on minor issues, not one where someone isn't willing to change, and certainly not willing to read a book like that, and instead demands the other just "love them for who they are".

> If it's like a critical body part, someone is way too dependent

This is unhelpfully extending the metaphor into a straw man. It's simple enough to see the point.

What about a situation where one party wants to make it work and the other doesn't?

In your example, your ankle is under your control - if you want to exercise it, you can. Your ankle doesn't actively sabotage your body (or if it does spread infection/etc, amputation is often used).

A marriage depends on two people working together to build a life together. If one person is not willing to put in that effort, the marriage fails.

Yes, it's a two-way street. That is why both partners say the vows.

This analogy would work better if organ transplants, prosthetics, etc. weren't a real thing and the medically right answer in many cases. If your hip needs replacement, no amount of eating healthier will cause it to no longer need replacement.

There are, of course, many situations where divorce is not the right answer for that situation, just like there are many situations for which a hip replacement is not the right answer. But it is within the set of potential right answers across all situations. There's no sense in trying to replace what is easily repairable, and there's also no sense in spending your life trying to repair what needs to be replaced.

And it's totally possible to go through life where neither you nor anyone you know ought to get a divorce - just like it's totally possible to go through life knowing nobody who needs a hip replacement, but that is hardly evidence that the procedure is the wrong answer for everyone!

The analogy works even better because organ transplants are sometimes necessary and sometimes not. Although GP started off defining a binary, I don't think they are thinking in black and white. A charitable reading of the GP implies the nuances you express.

I like this metaphor because it fits with the generation of my parents!

That's also the generation of people who don't go to the hospital because anything hurts. They go to the hospital 5 years later for something unrelated, when it's discovered it's 4 years too late to do anything about it.