The thing is, they're not even bad films, just some of them aren't about what people want them to be about. They're still pretty decent, but they lack what they had before Endgame. Outside of Spider-Man, Thor, & Guardians, people are like...who? The thing is, before Endgame people might have said that about Guardians, but they had built so much good-will, that people were down for it. Notice they didn't START things with GOTG or Ant-Man, right? The thing is though, it's not liek the Prre Endgame moveis are significantly "better" than the post Endgame ones - I mean a lot of them are the same movie, or at least the same formula. It's just, over that long of a time, interest can't be sustained when the people they want to see are gone.
The comic book industry seems to have to constantly learn and re-learn that lesson. The characters aren't replacable titles that can apply to anybody, it's about the characters. They are the important thing (and in movies, tha actors are important too!). Sure, you can tell a good story with a new person as Batman, but he'll never really BE Batman and fans will never be as interested as they are in Bruce Wayne. Sometimes you go along for the ride with the promise of a return to the status-quo, but that's not really possible in movies that operate in real-time with aging principles. Comic books can be good...until they're not. The same seems to apply to the MCU, which is the closest thing to comics we've ever seen in film. Look at DC? It's back to basics there! Is the lesson learned though? Only for now....