This seems incredibly narrow-minded, it is 100% possible for a couple to realize their dynamic as two people just doesn't work as well in a romantic relationship as it does platonically, and if both parties are emotionally mature enough with it, they can absolutely part ways and still maintain a healthy ordinary friendship post courting.
Asserting that such a phenomenon isn't "how reality works" seems like a weird projection of whatever personal expectations and hang-ups you have about how relationships work.
More importantly, please state your disagreeing opinion without the condescending personal attack smacked on.
And sorry if you feel that was a "personal attack" because I posited that such an arbitrary and unsubstantiated declaration about the reality of relationships might be coming from a mindset that doesn't give attention to the nuances and idiosyncrasies of relationships or the decisions individuals make in them.
So yes, growing apart does take place, but it is the effect not the cause.
A relationship by nature "disintegrates" if you don't work hard to sustain it.
But you are right that no one should be surprised, the outcome is as predictable as gravity and is bound by the same natural laws.