Most Marvel Studios phases in the past were building to the eventual Infinity Saga. This only needed specific characters to really pull off properly, which Marvel Studios and Disney had full rights to use. Spider-Man agreements were made later on but there were plans to use someone else if this did not work out. Now, we’re building up to the Secret Wars story arc. This is an absolutely gigantic story in Marvel Comics history that involves, we’re not joking, at least 100 characters. Plus, Doctor Doom and The Beyonder, key villains in the Secret Wars storyline, have yet to be introduced. This is why Phase 4 seemingly introduced us to countless new characters as well as introduced the multiverse overall.
Secret Wars essentially revolves around the multiverse folding in on itself. Beyonder, an all-powerful being, takes shards from all of these universes and places them together to create Battleworld. The heroes have to find a way to fix everything and defeat Beyonder. All while Doctor Doom is pulling some strings the entire time. The plan is to put this story out by 2025, so both Phase 4 and Phase 5 will need to introduce many characters. Plus, they will need to recast “certain” roles or perhaps keep them. All of the Marvel Studios phases, likely, built up to this massive event. To be honest, this is the true “End Game” for the MCU as nothing is bigger than this in Marvel Comics’ history.