Alien and Terminator have both had nothing but shit since their second installments.

If you look at only the movies, Star Wars is at least as bad after the original trilogy. Rise of Skywalker is bad enough to make me wonder if there was intentional sabotage going on.

But for my money, the best example of a franchise that starts strong and then turns to absolute garbage in the sequels is Highlander. The original is a classic that was never meant to have a sequel. Then it got a bunch of sequels, the majority of which have been frequently cited as among the worst movies ever made.

• Highlander 2 is a ridiculous betrayal of the original that wanted to be cyberpunk and retconned the immortals to being aliens, which just gets dumber the longer you think about it,
• Highlander 3 is an apology that erases 2, but also negates the original's ending for no reason other than to do the exact same plot over again... poorly.
• Highlander 4 erases all the movies and instead follows the TV show, killing the hero of the first movie as a poorly realized attempt to pass the torch, butchering the lore of both the movie and the show, and just being so awful that they chose to spend their time and money making deceptive ads with stuff that was never going to be in the movie instead of finishing the movie. Seriously, they released it with unfinished effects shots and missing scenes that were vital to understanding the plot.
• And then there's Highlander 5, which was a straight to Syfy channel movie that actually makes all the previous ones look good by comparison, Listing everything wrong with it would just be describing the entire movie.

Hell, the anime is the most faithful movie follow up, and it doesn't have any of the characters, takes place in a post apocalyptic wasteland, and introduces God, destiny, reincarnation, ghosts, druid magic, cyborgs, giant robot spiders, etc.