Avengers: Infinity War - The One Line That Confirms The Ending Won't Last
"There was no other way..."
If you came out of Avengers: Infinity War wondering how you were possibly going to go on with your life without a huge swathe of the biggest heroes in the MCU, you probably needn't worry about their deaths lasting.
Now, obviously, the Russos have promised that the stakes in Infinity War and Avengers 4 are genuinely real but that doesn't mean that Thanos' click won't be undone. It just means that some pretty big characters will be dead by the end of Avengers 4 (presumably joining Loki, Heimdall and Vision as the DEFINITELY DEAD crew).
Quite fankly, there was no way we were going to see a Phase 4 that didn't have Spider-Man, Black Panther, Doctor Strange or any of the Guardians given that they're the foundation of the first films of the phase (whether confirmed or assumed). So it would be reasonable to think that something big was going to happen in Avengers 4 to reverse the end of Infinity War - even without knowing what specifically will happen.
And if you look hard enough, there's actually a confirmation of the fact that the ending won't last in the film's final act, courtesy of Doctor Strange.
Strange's role in the movie is interesting as it's clear that he knows that Thanos' snap and the subsequent loss of half of the heroes is a necessary step to Thanos eventually being defeated. As he fades he makes it clear to Tony Stark that there was no other way and before that he expressly states that they are "in the endgame now."
You could assume that this is Strange simply acknowledging that sacrifices had to be made to save the universe in the long-run, or that Thanos initially had to win in order to lose in the endgame. But Strange knew that no matter how many were killed when Thanos clicked his fingers, there was no way that it would entirely last.
And how could anyone possibly know that? Well, because the Ancient One told him so.
Think about it - Strange says that he saw more than 14 MILLION futures and only one of them saw the heroes win. If the survivors are in the endgame and there was "no other way", this is clearly the future in which they won, but the very fact that Strange KNOWS of the victory is itself confirmation that his death (and those of his fellow heroes) simply cannot last.
Back in Doctor Strange's solo movie, we see the death of The Ancient One, who slows down time to elongate her final moments and says to Strange "I spent so many years peering through time... looking at this exact moment. But I can't see past it." That's how we know that she is done for - the fact that she can see no further than the point of her death.
If that's the case - and nobody with the ability to see futures (even 14 million of them) can see past the point of their death - then there is simply no way that Doctor Strange could see any victory over Thanos that required him to die as part of the plan. If Strange has seen that allowing Thanos to click and wiping out half of existence ultimately leads to victory, then he must be alive to witness it. In other words, he's coming back from the dead or he just plain isn't dead at all.
So, while we can't hope to know in advance HOW the snap is undone, there is fairly definitve proof that it absolutely will be. And when it comes to it, Doctor Strange will be there to witness Thanos' defeat.