Yeah, well it was the late 70's / early 80's. I shouldn't have made such a blanket statement, I suppose, but you just don't see such arrangements lasting long. At least, neither I nor any of my friends have...
But I think your observation is right on the money. One of the problems of a FWB arrangement is the involved parties' definition of friend! As the emotions shift, someone's bound to get hurt. You can set out all the ground rules you want, but everything changes, and it's rare that you can "go back to the way it was".
I married my best friend nearly 25 years ago, and we still are (married and friends). The romance has ebbed and flowed over the years - raising kids can do that - but we'll always be best friends, and that's a lot of love. Soulmates? Maybe. Can't imagine life without her. Well, maybe... especially after an argument.