It will be secure as long as they are both getting so much out of the relationship.
Now suppose their circumstances change so that this is no longer true, in some asymmetrical way; e.g., one partner is seriously injured in a car crash and (e.g.) requires care that's burdensome to the other, or suffers brain damage that changes their personality, or is disfigured and loses the physical attractiveness that was important to the other partner, or something.
At this point, it is no longer true that both partners are getting a lot out of the relationship. The still-healthy partner would (aside from any feelings of obligation they may have developed, which if I'm understanding the usage in this thread correctly should not be considered part of a truly "transactional" relationship) be happier without the maimed partner. In a purely transactional relationship, the maimed partner gets thrown out at this point.
That may indeed be better for the still-healthy partner. It's clearly worse for the maimed partner. And it's at least plausible that on the whole it's better for us all if the usual practice in such situations is not for the person who just got maimed in a car crash to be discarded and left to fend for themselves somehow. And I would guess that most of us who are in long-term relationships hope that our partner wouldn't do that if we suffered some such disaster.
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/...
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.
Notes for future OT posters:
1. Please add the 'open_thread' tag.
2. Check if there is an active Open Thread before posting a new one. (Immediately before; refresh the list-of-threads page before posting.)
3. Open Threads should be posted in Discussion, and not Main.
4. Open Threads should start on Monday, and end on Sunday.