I suspect I’m not the first person to realise this but I’ve realised there’s a fundamental flaw in the Back to the Future trilogy.
In the second film, Doc explains how the timeline works and, in particular, how if you change it, it creates a new future timeline on which you then travel.
So, when in the 3rd film they go back to 1885 and they change the future (the ravine, etc), this means they create an alternative future. A future in which Marty didn’t go back to 1955 and change how his parents meet. So when he ends up, at the end of the film, in 1995 doesn’t he now have the dorky parents and siblings again?