Why do you think the author ends by describing Pip and Estella walking hand in hand – “as the morning mists had risen long ago when I first left the forge, so the evening mists were rising now”?

You are correct, there was supposed to be another ending. It was far less romantic involving Pip discovering that Estella had, after Drummle's death, married a country doctor from Shropshire. He does see Estella but they basically shake hands and part. Dickens changed the ending at the urging of a colleague. He also thought a happy ending might resonate more with readers. I am inclined to agree with many critics that this ending seems out of place with the tone and mood of the rest of the novel. Pip undergoes many struggles and all of a sudden Estella is found wandering the garden ready to start a life with Pip. Perhaps I'm being a little pessimistic though. Still, Pip comes to certain conclusions. He realizes that social standing is not the key to happiness. He also understands that the world changes. This can be seen by the destruction of Satis house which, like the clocks, was stuck while the world changed around it.