I think Thor already knows they’re going to lose against Doom
In Infinity War, Thanos told Thor that he should have gone for his head, and because Thor missed that opportunity, Thanos’ plan ultimately succeeded. Fast forward to Endgame, Thor remembers what he was told and ends Thanos’ life by going straight for his head.
Now, when we look at the Doomsday trailer, Thor continuously says that the OG Avengers were stronger than the current roster and that the new heroes would die facing a lesser threat than Doom. We also know that, even though the new teams are incredibly strong, they aren’t the same as the OG Avengers.
In one part of the trailer, Thor confronts Doom on his own and ultimately loses. Maybe this happens early in the film, before the rest of the heroes have even had a chance to face him. This could be why Thor seems to already know that they’re going to lose. He’s the strongest Avenger, and if even he can’t stop Doom on his own, then he knows exactly how bad the situation is.
Thor probably isn’t just warning everyone because he thinks Doom is powerful. Maybe he has already fought Doom and experienced firsthand how completely outmatched he was. That’s why he seems so certain that the new heroes aren’t ready for him and that even all of them together could still lose.
It would also explain why Thor keeps comparing the current heroes to the OG Avengers. He knows what the original team was capable of; he fought alongside them, and he knows that if even the strongest Avenger can’t beat Doom, then this new roster is going to be in serious trouble.
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