The Matrix is an often used example, but for me it’s the Alien Prequels - especially Alien: Covenant really makes the Original Alien much worse. When the original was released in 1979 it had the perfect Monster. A dangerous killing machine of unknown origin. The missing background of the alien is a big part of its scary mess. It’s a blank space in its mythology that the viewer can fill with many explanations. As these explanations are not precise they don’t have to be logically coherent.
Covenant (and to a lesser degree Prometeus) wanted to fill this blank space and tell us the aliens origin. But once you fill out this missing piece of information it is fixed and can only be one piece. There exists now only one singular explanation. And its a boring: The Xenomorph is basically a creature with it’s origins on earth (because David, who’s origin is on earth created it).
I find this hugely dissapointing. The biggest dangers of deep space are all human in origin is extremely small minded.
(Star Trek: Beyond had the same boring plot - the mysterious villain turned out to be a human after all. As if only humans are capable to pose (or create) a serious thread to humans.).
What are your examples for franchise-movies that somehow made the original worse?
By retroactively changing elements of the established story later on, which ruins the context when going back to the original film. As OP points out in the body text of their post.
But that changes you, not the movie.
I think the point is that when a new movie is made, many people put them together so that one informs the other, in the same way that the first half of a movie informs the second half.
it’s relatively easy to imagine a movie that starts great, but the second half introduces a major twist that makes the first half seem dumb.
For sequels/prequels we have the ability to just say “Well that one isn’t canon”, in a way we can’t for a self contained movie. But some people (including me) have a hard time with that, because it seems so arbitrary. The world is no longer something I am experiencing and enjoying, now it is something I am controlling and curating, which hurts immersion.