If you have seen Avengers: Infinity War, you already know that the film doesn't end particularly well for earth's mightiest heroes. Thanos uses the power of the Infinity Stonesto wipe out half of life in the universe, creating a massive cliffhanger that will need to be answered next year. There's a lot of debate over how Thanos could've been defeated, but a new theory suggests that the entire reason he waited to attack Earth in Infinity War was that the more powerful MCU characters had finally perished.
Not only has the Marvel Cinematic Universe been building to the arrival of Thanos, but this theory even suggests that he waited to attack Earth in 2018 because the more powerful characters like Doctor Strange's The Ancient One and Guardians of the Galaxy Vol. 2's Ego had finally been taken out of the equation. With these power players gone, Thanos could barrel through the MCU's primary heroes.
It's an interesting theory to note, because it actually echoes some notable conflicts that have happened in the pages of Marvel Comics over the years. Specifically, the original Infinity Gauntlet storyline, Thanos goes head-to-head with the Celestials while wielding the titular golden glove.