Well... What made it such a success to begin with? I'd contend that it was the simple formula of having interconnected films that built up to a big event film every few years.

Phase 4 abandoned that approach (in my opinion because they shifted from building up the MCU, to using the MCU to build up Disney+). Black Widow, Eternal, and Shang Chi were self-contained outside a few references/cameos. Multiverse of Madness had connections but it wasn't clear what it was building up to (aside from a new Disney+ show)... Maybe the event film after the next one? And so on and so on. The new Thanos appeared first in a Disney+ show, meaning that casual audiences were no longer in the loop. Same with Wanda's heel turn. Once they didn't understand what the MCU was doing they stopped showing up.

Of course, losing so many actors at the same time hurt too.