In case you didn’t realize, spoilers.
Five years ago Thanos reached his goal and managed to make half of the living sentient beings in the universe just vanish.
What happened during those five years? I mean, think about it. If half the people were gone, what would happen? Mass slaughter of animals, redistribution of population, when many towns would be completely abandoned as unneeded and production of pretty much everything, but especially food, would go down, as there wouldn’t be a need for everything.
People would adjust. We always do. We would have to shut down many plants that would be dangerous if left running. On the other hand, this would also allow us to halve all the emissions of pretty much everything.
On the other hand, it would also allow for those without food or housing to gain some. There would enough empty real estate to give apartments or houses for everyone and the mass slaughter of animals would help feed everyone.
Than, five years later everyone is back. Oh, shit. Many houses have been abandoned and are thus very rundown. Food production has been cut down immensely. There just isn’t enough crops or animals for everyone to eat. This doesn’t get fixed immediately either. Food can’t be produced as easily as something like electronics. It needs time to grow. Crops take months to be ready (at best) and many animals take years.
There are wars being fought over food right now. What would happen if we had 3.5 billion new people to feed?
So, while Thanos might have been wrong in what he did, the Avengers spited him in the worst way possible by triggering the very situation Thanos was trying to avoid. Of course, we only see what’s going on on Earth, but as Captain Marvel explains in the beginning, this is not the only world where this happens. This is going to take place on an universal level.
Actually, this might make it even worse for backwards planets like Earth. Suppose you have billions of extra people you suddenly need to feed? Well, there is that one place we can pick it up from. Sure, there’s some alien race there, but that never stopped even us humans from stealing real estate from each other. Why would someone of another species care?
So, when the aliens turn up to steal all our food, you know who to thank.