Seems to me that we need to differentiate between relationships in RL and relationships in fiction. Relationships in fiction are part of a larger picture and make a point. H/G is as much about who Harry truly is as a character, and who he is becoming as he matures, as it is about the appropriateness of that relationship itself. Similarly, R/Hr is clearly (in my mind) about recognizing incompleteness in onesself and the attraction of the other -- Ron & Hermione each pull Harry in opposite directions, which allows us to see Harry's mind working in dialogue instead of internal monologue (a very old device). To have them end up with each other is to say that neither extreme can live without the other to nourish it.

In RL our relationships don't have to satisfy thematic or symbolic purposes, so no, we often don't end up with our High School sweethearts. We mature and we need other things. :)