I have been a huge fan of this movie all my life (I was about 2 week old when it came out in theaters and at 4 years old I literally watched it on tape every other day) and I never thought about the ending in that way. When put like this, I can see how some might see the ending as dark. Yes now Marty has to adjust to a different family dynamic but I think mostly it is a better family. Not because they have more money, because to me, the whole family is closer. Starting with the parents, George is more present and engaged with his whole family, Lorraine is no longer a drunk and in better physical health and both are happier, and the kids are even more connected with each other. In the end Marty was able to save himself and his siblings from being erased from existence and the family is whole and together. Don't forget, this is not the end of the story, and we never get to see the final results of all of Marty's time traveling and what effects it might have had on his family dynamics.