I disagree. I am very saddened by the common stereotype "if there is a couple already formed from the start it should NECESSARILY be ruined during the story for the sake of character development", as if there were no other ways to develop characters apart from splitting couples apart. I admire very much those characters who stay together forever, especially when they are childhood friends. Unfortunately, such stories are so rare, in the modern fiction there are always love triangles, cheating, polygamy, multiple marriages, etc. It depresses me, seriously.
As for Ran, I would really like to have a friend like her in real life: open, honest, with a good heart. And I would never be able to trust Haibara.
I am not forcing my opinion on anybody, but still...