The first three phases of the MCU were all building toward the humungous movies of Avengers Infinity War and Endgame, which brought together all of the heroes established over the eleven years prior.
After those two films - and their epilogue, Spider-Man: Far From Home - the MCU could have conceivably ended. All the important loose threads had been tied up, the two principle protagonists (Tony Stark and Steve Rogers) had received fitting endings, and the universe was saved.
As a finale to twenty-three movies, that's a pretty amazing success - and so matching such an achievement will be just one of many problems for the cinematic universe going forward.
The stupidly high profits Marvel movies make obviously prevent Disney from ever ending the venture, meaning the adventures of the Avengers and co. will probably continue on the big screen for at least another eleven years.
Even with the uber successful formula that has been established, the talent the MCU is able to attract etc., the post-Endgame road isn't a completely smooth one. Here are the potholes awaiting Marvel Studios as they venture into their next big project...