There are always alternative endings, whether you believe in parallel universes or not. The parallel world has long been a staple of both TV and literary SF and of course there’s the whole genre of alternative history fiction. I always loved “What-ifs” ever since I watched the Doctor Who story “Inferno” as a child wherein Jon Pertwee faces off against the Brigade Leader, an alternate version of “our” universe’s beloved Brigadier.
Well, truthfully I didn’t understand it the first time I saw it, as I was way too young, but that, and the Star Trek episode “Mirror, Mirror” were my first exposure to the idea. Then there were the “What-If” takes in Marvel Comics – what if the Fantastic Four had different powers, or what if Doctor Doom had become a hero instead of a villain…? I loved that stuff. I soon became aware that you could apply it to yourself – what if I chose this way over that?
As you get older, you realize that the power of choice is what dominates us as human beings – it comes into everything we do. It’s the crux of the argument between predestination and free will. The power of conscious choice is what makes us different from animals (along with the conscious knowledge of inevitable death, but let’s not go there).