If you think about it, we’ve known that Thanos was the final boss the heroes would be facing at some point since the mid-credits scene in 2012’s The Avengers.
The Infinity Stones have been the through-line of The Infinity Saga and everything has built toward that Thanos encounter.
There is also something to be said for keeping fans in the dark and maintaining an air of mystery about what the Marvel Cinematic Universe will be moving forward.
Because we really have no idea what this universe will look like post-Endgame.