Suppose she gets hit by a bus and is now disabled. You calculate that she is no longer a good investment. Do you shrug and write her off as degraded capital?

If circumstances changed sufficiently such that she was no longer a good investment, of course I would end the relationship. Being hit by a bus wouldn't do it, but I can imagine other things that might.

A healthy attitude to a relationship makes the other person an end in herself.

I agree with Caue below; that seems to be the opposite of a healthy relationship.

And not just an unhealthy relationship, but a meaningless and unstable one too. If I really did view my fiancee as an "end in herself", that would mean I wanted to make my fiancee happy for no reason. Why isn't my terminal goal making some other girl happy? Indeed, why isn't my terminal goal making her sad? Or polishing rocks? No reason? This is absurd. And if making her happy is my goal for no reason, who's to say that goal won't switch tomorrow? Our relationship would be as fragile as my fatuous goals. Frankly, I am horrified at the thought of being in a relationship with someone so psychologically imbalanced as to want me to be happy for no reason.

I don't think people really do have "ends in themselves," we aren't like paperclip-maximisers. All our ends are explicable in terms of our other ends, in a complicated tangle. Yes, I want to make my fiancee happy. But I want to make her happy because it deepens our relationship, and makes her better disposed to me, and provides insurance against some time when I screw up in future, and so on.