Simple, the demographics have shifted. The MCU is not bringing in the younger audiences like it once did. And there are other factors. I don't think Feige ever really considered the result of having an "ending", and didn't know how to build back interest.
Same thing happened to WWE once the Attitude Era ended.
Steve Austin retires and The Rock becomes a movie star. Both are gone. Tony Stark dies and Steve Rogers retires. Both are gone. In both cases, no new stars are made to replace them.
New stars are attempted. There is also an attempt to take b-players and elevate them to main characther status. In both cases, those attempts don't work.
In both cases, fans become disinchanted with the companies and their way of doing things, and goodwill turns into bad. People lose interest.
WWE brings in Goldberg, Scott Steiner, and the NWO, all from WCW, and revive ECW, all to bring the nostalga. Marvel brings back the FOX X-Men for a couple of movies and the other two Spider-Mans for No Way Home to do big box office, and keep things afloat. In both case, the comon narrative from fans is that Marvel/WWE can't create new stars anymore.
The paralals are striking.