No relationship is secure against any and all changes.

For the avoidance of doubt: I agree, and I was not in any way making the argument "I can imagine a situation in which a transactional relationship would be imperfectly secure, therefore transactional relationships are bad". Rather, it was: "It seems like in many quite common situations a purely transactional relationship might be less secure than we would like our relationships to be, where a not-so-purely-transactional one would be stronger in a way that's probably better overall".

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?

The latter, obviously. But (1) it's by no means only permanent brain damage that leads to the kind of situation I described and (2) I don't see any reason to think that a purely transactional relationship is more secure against passing fancies than a not-so-purely transactional one.

And that there's something bad/unromantic/unacceptable if I do.

Bad, yes (in the sense that the policy of abandoning your partner in such situations generally produces net harm and that we'd all be better off if it weren't generally adopted). Unromantic has nothing to do with it (except that if "romantic" is one opposite of "transactional" then the unromantic-ness of abandoning your partner might make less-transactional relationships more secure in such situations). Unacceptable, meh, I dunno; I don't see any reason why you should care whether I accept what you would hypothetically do in that situation or not.

So I don't think non-transactional wins over transactional here.

I'm not sure I understand your reasoning. Perhaps the following questions will help: Do you agree that, other things being equal, a relationship in which neither partner would abandon the other in such a situation is probably a better one overall? What sort of qualities would make a relationship have that property? Are they more or less likely in a purely transactional relationship?

If you truly cared about your partner "as an end in herself" you still wouldn't leave. Care to bite that bullet?

I'm not sure exactly what position you're arguing with and why you think it's my position, but: if my wife (I do, as it happens, have a wife) were unfaithful or became addicted to drugs, I would not necessarily want to end our marriage on that account. I would much prefer to salvage if it possible. (Violence? Not sure. We have a child and keeping the child safe would be important.)

I disagree fundamentally about obligation [etc.]

OK, then maybe what I've been understanding by your use of the term "transactional" is different from what you've been meaning to say. I've been assuming it means roughly what knb seems to have meant in the earlier thread (this comment and its grandparent), though actually I'm not sure that that's enough to pin down the relationship between transactionality and obligation.

This

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.

does definitely seem to indicate a difference in meaning, though; I don't see how "non-transactional" implies "everyone was acting purely for their own ends" any more than "transactional" does.