In recent years, three "sequels" came out that were all commonly viewed as worse than the earlier works: The Hobbit trilogy, Season 8 of Game of Thrones, and the Star Wars sequel trilogy.
Sequels are frequently worse than originals due to regression to the mean, and I think there's some truth to peoples' claims that part of the problem with these in particular was that people viewed the originals when they were younger and therefore saw them through rose-tinted nostalgic glasses.
On the other hand, sequels aren't destined for failure. Even just within these three franchises, the first several seasons are widely viewed as equal to the first and Empire Strikes Back is widely seen as better than A New Hope.
[The three Lord of the Rings movies are generally viewed as equals, but they were all made simultaneously, so they don't really count as sequels for this discussion.]
There are plenty of concrete arguments about why these sequels are worse (and better) and I don't have a whole lot to add to those. Instead, I have a broader observation regarding what caused these sequels to be generally worse that I haven't seen anywhere else: all three of these sequels were rushed.
Writing the initial Lord of the Rings took ~13 months and it was largely thrown away. TODO. Preproduction lasted
The Hobbit was originally going to be two films directed by Guillermo del Toro, with Peter Jackson being a consultant. However, after ~16 months of pre-production, del Toro left the project, and around 4 months later, Jackson became his replacement. After 5 more months of pre-production, filming began and lasted 9 months. 7 months later, Jackson decided to make a trilogy instead.
The Star Wars Sequels swapped directors between each movie and (iirc) changed back to Abrams on 9 pretty late. All the while they pumped one out each year. D&D wanted to wrap up Game of Thrones quickly to move on to Star Wars even though HBO wanted more season. In short, in all these examples directors gave the movies significantly less time than they used to and the quality went down.