Pretty sure it's Thanos, because ever since he died (twice!) the franchise has been in limbo. Interest in the MCU has declined dramatically since the Infinity War arc ended.
Even with all the new creative shows they've released since then, the MCU is still struggling. And I personally think all the TV series from WandaVision and on have been much better than the older series.
EDIT: I guess the same could be said for Stan Lee. He died near the end of the Infinity War arc and the MCU hasn't been the same since.
Avengers: Infinity War was probably the best MCU film so far. Then Stan Lee passed away, and whereas Avengers: Endgame gave us a resolution to the conflict of Infinity War, it just doesn't feel on the same level emotionally as Infinity War. It resolved the plot, but it just wasn't quite as satisfying. The rest has been downhill since.