But they might have built a better one too. Two "swinging couples" might just be two dead relationship that survive because the partners in them simply don't have it in them to put an end to such a lon relationship officially even though the feelings are gone. It's pernicious to look at things like that only in what is lost instead of what is gained.
And two swinging couples might be perfectly happy with their relationships, but want to play in some way their partner isn't into. Maybe the play takes pressure off a spouse and makes everyone happier.
There is a saying in the polyamorous crowd: Your Mileage May Vary: YMMV. People are varied enough that the only way to know what is going on is to ask and communicate. Decide on ground rules and stick to them, only re-negotiating when emotions are not on overdrive.
Communicate, communicate, communicate.