Endgame Theory: Captain America Fixed Every Timeline Except One
At the end of Avengers: Endgame, Captain America returns to the time stream in order to return the Infinity Stones and fix every timeline - but he failed to fix the one Loki escaped into. The biggest movie of all time, Avengers: Endgame was the final capstone to the Marvel Cinematic Universe's Infinity Saga, seeing the Avengers come together to defeat Thanos and his army once and for all. The surviving heroes from Avengers: Infinity War went back in time to steal the Infinity Stones from throughout their shared history in order to bring half of all life in the universe back from the dust in their present.
Having been warned by the Ancient One about removing Infinity Stones from their respective places in time and space, Bruce Banner promises to have them returned to each and every point from which they were taken, thus preventing the splintering of reality into dark, divergent timelines. In the closing moments of the movie, Captain America does just this; being sent back in time by Smart Hulk, with a briefcase full of Infinity Stones by his side and Thor's hammer in his hand, in order to return the Stones to every location they came from. While Captain America completes his mission off-screen - sadly robbing fans of a reunion with Red Skull - audiences know that he is successful due to his sudden appearance on a bench nearby, albeit as an old man.
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Following the seeming success of his task, the super soldier decided to return to the post-war years and find his one true love Peggy Carter for the dance they were promised. He says to Falcon, "Well, after I put the stones back...", so viewers know that Captain America returned all the Infinity Stones to where they were obtained - including the Space Stone, which he and Tony had procured from Camp Lehigh in 1970. This, however, doesn't account for the consequences of the Time Heist in 2012, which saw Loki escape with that time period's Space Stone after Tony Stark is taken down by a Hulk enraged by a stairwell. Here's why Captain America may have fixed every timeline except one.
As audiences know, Captain America's plan was fix every one of the timelines the stones were taken from. That meant going back to Camp Lehigh in 1970 to return the Space Stone; Stark Tower in 2012 to return the Scepter (containing the Mind Stone); the Sanctum Santorum at the same time to return the Time Stone; Asgard in 2013 to return the Aether (the Reality Stone); and Morag and Vormir in 2014 to return the Power and Soul Stones, respectively. While Cap tells Falcon that he returned all the stones when he appears as an old man in 2019, audiences don't know that he was actually successful in repairing all the timelines.
If there was a mess made by his reverse-Time Heist, the closing moments of Avengers: Endgame were not the time to address them. Rather, Marvel would utilize them for a future, then-unknown project for their impending Phase 4 of the Marvel Cinematic Universe. As far as Captain America within the storyline was concerned, he had to return the Space Stone to 1970, most likely not thinking about the fact that Loki had escaped with a Space Stone after Stark bungled the heist in 2012. With the knowledge that Loki has a Disney+ show - the self-titled Loki due out in 2021 - centering around the escaped 2012 Loki, it is safe to assume the broken timeline will be addressed there.
Every other branching reality threatened by the removal of the Infinity Stones - as warned by the Ancient One - would have finally been fixed when Captain America successfully returned those six stones to whence they came, restoring harmony and balance to the universe. This, however, does not account for the fact that in 2012, that time period's Loki escaped with its Tesseract, which - unbeknownst to all present - contained the Space Stone. When 2012 Loki grabbed the Tesseract off of the floor of the lobby of Stark Tower, he promptly disappeared into a portal (much to Thor's dismay). Even if 2019 Cap knew to return to 2012 to track down Loki and that Space Stone - and it appears that wasn't on his time travel agenda - the likelihood of finding the trickster for a lone Avenger adrift in time could be slim to none. This means that the 'Loki timeline' cannot be fixed - by Captain America, at least.
Avengers: Endgame directors Joe and Anthony Russo, in an interview with Business Insider, were very candid about what 2012 Loki's escape means for that timeline;
"The intent was that [Captain America] was going to correct the past timelines at the point that the stones left. Loki, when he teleports away with the Time Stone, would create his own timeline. It gets very complicated, but it would be impossible for [Cap] to rectify the timeline unless he found Loki. The minute that Loki does something as dramatic as take the Space Stone, he creates a branched reality."
With the information available, it's clear that Loki's 2012 escape in Avengers: Endgame did create a branched reality, one that Captain America could not prevent for lack of resources or knowledge. The news has already been released that Loki's six-episode Disney+ miniseries will follow the 2012 iteration of the character as he slides through time "influencing human history", being pursued by an authoritative body known as the Time Variance Authority (whose job is to protect the timeline from being damaged).
At one point, he is detained by the TVA, so there is hope for this splintered, branched reality yet. Loki is also intended to tie into Doctor Strange in the Multiverse of Madness, leading to speculation as to whether 2012 Loki and his branched reality is the cause of the titular Multiverse of Madness, which implies that the Time Variance Authority may ultimately be prevented from stopping Loki. Time will tell when Loki hits Disney+ in 2021.