Which just makes things worsething about time travel they establish in this movie is
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If the time-travel involves travelling to alternate timelines, then Captain America et al are essentially stealing from those time-lines. "Sorry alternatve timeline guys. We have a problem and will solve it by making sure you get something at least as bad."
The movie makes no attempt to have the time travel work with linear causality. But, inter-timeline theft.... Wow. The Avengers are assholes.
That is far too much to leave for the audience to assume. There should have been some kind of reasoning on screen to make this work, not a statement in an unconnected interview.Spoiler
the Russo's explain that Cap had to make another jump, back his own universe, after he had lived his life with Peggy.
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s the second Thanos and Gamorra that throw things off for me. Maybe when I get around to watching it a second time I'll piece things together.
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There is also another problem with the movie's resolution.
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Assuming that humanity did not catch a statistically unlikely break or burden, Thanos killed ~half of the human population. The population would have been reduced to ~4.5b (the global population of the mid-20th century). While live-stock and plants would have been similarly reduced, the demand on agriculture (and ecosystems as a whole) would have also been reduced.
In fact, ecosystems would benefit, as the amount of available land would have effectively doubled. There would be a genetically viable population. And, labor would be twice as valuable, even with technology making many jobs redundant. (Frankly, Thanos' plan is hardly the world thing one could do with the gauntlet.)
There would be some disruptions (beyond the obvious psychological impact for the survivors). But, things would actually work out pretty well in the long run. After 5 years, the world would probably have achieved a new degree of normal, with human commerce and agriculture having less of an impact on remaining wild-space. (Plants tend to regrow pretty quickly when left alone. Look at poorly kept yards or parks. Similarly, any species that managed to avoid a genetic dead-end would also recover nicely, having less conflict with reduced numbers of people.)
Then, after all that, imagine the population doubling in the space of a day. Infrastructure and food production would have to double capacity to match the added demand. Everybody would have to give up half of their backyard, because the available land would be cut in half. Everybody's standard of living would be reduced.
Put another way, undoing what Thanos did would actually make things worse.