So after seeing it again, and reading a few articles about it. Here's the gist:
They establish the rules of time travel in the movie. As time travel doesn't actually exist (shocker!) - the movie can play by whatever rules they care to follow, as long as they follow it. In this case, they don't follow the usual tropes that Back to the Future does, aka The Grandfather Paradox.
In the film, Hulk clears it up by saying there is no future traveling. We can only go back, not forward; and if you go into the past, it is now your future, and the present is now your past.
If anything is altered, it creates a branched timeline, or an alternate reality - which is why they have to replace the Stones back to where they stole them. Basically, the MCU is the "Prime Timeline", or in the comics, known as Earth-616. When Loki gets the Tesseract in 2012, it creates a branch but it will not affect the Prime Line.
In the film, Hulk clears it up by saying there is no future traveling. We can only go back, not forward; and if you go into the past, it is now your future, and the present is now your past.
If anything is altered, it creates a branched timeline, or an alternate reality - which is why they have to replace the Stones back to where they stole them. Basically, the MCU is the "Prime Timeline", or in the comics, known as Earth-616. When Loki gets the Tesseract in 2012, it creates a branch but it will not affect the Prime Line.