I've seen so much back-and-forth over the last few pages. Short version: There are problems with the time travel in this movie. And I'm willing to bet the creatives are aware of them. I am 99.9% certain fallout will continue into Phase 4. Both the Loki streaming series and the next Doctor Strange will almost certainly be related to this.
Breaking it down:
• Thanos split the Sakaaran ship in half. At the beginning of IW, we saw half the ship drifting out-of-frame, while Thanos and crew were dealing with folks on the half closer to his own ship.
• Structural technicalities of the Infinity Stones... As has been pointed out, it's not as easy as "just taking the Stones back when/where they were taken from". The Space Stone has to get remade into the Tesseract, the Reality Stone needs to get re-"liquified", the Mind Stone needs to get a new scepter gem to house it... Not even getting into other issues.
• Alternate timelines -- what the Ancient One warned about: Merrily dancing across the paradox minefield, it's impossible to replace the Stones without any lasting effect, due to how the Stones were got in the first place. There was a lot of Battle of New York stuff that played out differently between the 2023 folks showing up and when they left again. Cap had no memory of fighting himself during the aftermath of the Battle of New York. Sitwell and Rumlow didn't think Cap was secretly HYDRA in Winter Soldier, not to mention Loki absconding with the Tesseract. Same with Past Thanos coming forward -- as soon as the Nubulas mentally synced, Thanos' timeline split off from what it had originally been, even though it couldn't in order for what happened first time around to have happened. Loki taking the Tesseract, alone, has massive ramifications. Not imprisoned on Asgard where he can assist the Dark Elves, no Tesseract in Odin's Vault to spur them to store the Æther somewhere not on Asgard. Which impacts the fate of the Collector and the Guardians. Also, given Loki vanished when Thor was right on the brink of hauling him back to face judgment, I can't see him leaving again until he finds his brother. And so on and so on.
• Cap. I'm trying to find the time to re-watch Captain America: TFA, Avengers, Winter Soldier, and Civil War. I'm seeing people pointing out Peggy never named her husband, but I clearly remember a scene where someone was telling Cap who she married, and him saying "I knew him. Good man." I can't see Mr. Decent depriving him of his happiness with Peggy, or hers with him, or their two kids their existence, just for what he'd see as his own selfish desires. And I don't accept that guy dying and Steve taking on his identity, because I can't see him "playing dead" and leaving Peggy to her Alzheimer's alone. I can see him going back and getting his dance with Peggy and both of them being aware of his duty to history to not meddle, and then him leaving again. But that would interfere with her emotional recounting of losing Steve in WWII in the documentary footage. Whatever the case, though, I cannot see him living out his life without getting involved. Sitting back, knowing HYDRA is infiltrating SHIELD and doing nothing about it? Knowing Bucky is being tortured and brainwashed by the Russians and not mounting a rescue? And, as a side benefit, shutting down the Winter Soldier program and saving Howard and Maria? As the other alternate timelines were superimposed on the "real" one, so, too, must Steve's new backstory.
• Cap II: One of the less-problematic Stones to return would be the Soul Stone, but his reaction to the Guardian would be too important to have gone so unseen, and the impact of his returning the Soul Stone is... fraught. Would it bring back the one who was sacrificed to get it in the first place? Of all the questions about the process of returning the Stones, that one is the one most needing to be shown.
My takeaway: If these alternate tributary timelines -- Thanos and crew, Battle of New York, Camp Lehigh in 1970, etc. -- are all interwoven with the main timeline and at the same time the changes made in them don't impact said main timeline, I feel like there's gonna be some temporal fallout needing dealing with. They spent too much time talking about time-travel problems in dialogue for me to accept the writers and directors then were oblivious to the issues they were creating in those exact terms. These very questions -- Steve interfering in his own past and Peggy's, Loki and the Tesseract, what happened when Steve took the Soul Stone back to Vormir, Vision's fate... I feel like the starting thread of Phase 4 is sorting out all that.
That aside, brilliantly done. Excellent drama and character moments. Good action and organic humor. I am satisfied... but waiting. This is the end of the beginning, and there's more to come. Spider-Man: Far From Home is going to be the last film of Phase 3. And, so far, nothing has been announced going forward, beyond the vague tease of GotG3. Not what films, or when they're coming out. The Netflix series have ended, and there's a two-year moratorium on even beginning production on those properties for Disney+. The new series for Disney+ have been announced, but no premiere dates yet. I kinda hate the lack of anything to grab onto.