Since brevity is the soul of wit, I will be brief:
As one of Clark-Flory’s interviewees (her one-time FWB) pithily says “I’ve been in so many of these situations and, basically, they work until they don’t.”
And that is the point.
All relationships, whether they are serious/monogamous/long-term, open, sex-only, sex-free, or entirely platonic, work until they don’t.
People change. Sometimes changes affect compatibility, sometimes they don’t. Sometimes people decide to work through changes together, sometimes they don’t, for an infinite number of reasons or no reason at all.
To pretend that FWBs and NSAs are the only relationships subject to human mutability is fatuous.
image from the Telegraph.co.uk