No relationship is secure against any and all changes. That's absurd. If the universe undergoes heat death, marriages will suffer. But see above for why transactional ones are more stable than non-transactional ones. Which is more common, permanent brain damage to one party in the relationship, or one party in the relationship having a passing fancy for someone else?
I think the intuition that you're getting at with your car crash example, as Caue says above, is that I shouldn't want to leave her in that situation. And that there's something bad/unromantic/unacceptable if I do. But if I still want to stay, we haven't left a transactional relationship at all. The response "Yeah, in those circumstances, the time I spent with my partner would be nightmarish, but I'd stay with them anyway just to make them happy" is equally bad/unromantic/unacceptable. So I don't think non-transactional wins over transactional here.
You also bias the question by the type of change. I think it's no coincidence that you and knb both choose the example of a vehicular accident, where the injured party is presumably innocent. How about if one of the partners is unfaithful, or takes to drugs, or violence, or whatever. If you truly cared about your partner "as an end in herself" you still wouldn't leave. Care to bite that bullet?
Incidentally, I disagree fundamentally about obligation - that's not outside transactional relationships. Indeed, binding your future self is the key to most transactions. If you exchanged vows to stay with the other party in sickness and in health, and then the other party gets sick, you should have to stay (or pay damages) if they get sick. You made a transaction, and you should have to stick to it. Obligation, on the other hand, doesn't seem to have any place in a non-transactional relationship; as everyone was acting purely for their own ends to begin with, there can't be any debts or obligations. So if people change their minds, they can just waft out of the relationship and abandon their partner. Yet another reason why a transactional view of relationships promotes stability.
You could make the argument that someone in a relationship in which serious changes have happened should precommit to keep in the relationship even if it changes. The precommitment is bad in the case of some changes, but makes the relationship more stable and reduces the chance of there being such changes in marginal cases (such as one partner becoming incrementally less attractive and the other partner having an incrementally greater chance of cheating on the first partner).
Doing things out of obligation, even though they don't benefit us, is just our wa...
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