A nice time travel premise, in theory....

Rating: 6/10 (29 April 2019)

There are already a surfeit of general reviews for this movie, so I am going to focus on the central conceit of the plot - time travel - and the way it is handled.
The time travel device is flagged with the tongue firmly in the cheek, as the characters break the fourth wall to share jokes with the audience about how all our time travel knowledge comes from... movies. Therefore it is not knowledge at all, and we can freely revolutionize our sense of time travel on the fly. Yay!
So the old logic of time travel - that you change and endanger the present by messing with the past in any way - is done away with. And a new logic of time travel is introduced: now changing the past in no way affects the present, instead just creating new parallel timelines. Or, to put it as the Avengers' writers do, the 'past' that I visit is actually in my future. It is not even really happening in the past...(?)
Thus, to use (and modify appropriately) the old Back To The Future premise, if I go back and tell my past self the outcome of a bunch of sporting events and advise making a bet or two, I will not be one penny richer when I get back to the present, because that is not really my past self. It is my past self from another timeline, one who got rich using a list of sporting results from the future. Get it?
I must say that when Avengers:Endgame proposed this premise I was initially very happy with it. Honestly, it makes a hell of a lot more sense than the normal time travel logic, for one simple reason: if there is only one timeline (as in most time travel movies), time traveling to the past and changing it is highly likely to prevent the time travel taking place, certainly in the way that it did in the initial, non-past-changed version of the present.
So for instance, if I go back and kill Thanos as a child (as Ant-Man suggests), he never gets the stones, I am never prompted to time travel by him killing half the universe, and the time travel never happens, so who exactly went back to kill Thanos? Certainly not the version of me in the present in which Thanos is dead! When you think about it, there MUST be two timelines: one where Thanos had the stones and, in desperation, I went back in time and killed him; and another where I was chillin', because I never heard of Thanos, because he was dead. Simple, right?
Why, then, I hear you cry, did most if not all previous time travel movies not notice and obey the clearly superior logic of time travel - namely that changing the past cannot affect the present - followed by Avengers:Endgame?
The answer is fiendishly simple: If time travel creates alternate timelines rather than changing the present, it also creates no possibilities for dramatic jeopardy, because nothing I do in the past makes much difference. And this is true not just for the present, but also for the past. Are you with me? Allow me to explain...
Say I go back to the past - not far in the past, just five minutes - and empty my own wallet, which is on the kitchen table (let's say it contains a 20 pound note). I now return to the future with 20 quid in my hand, but when I go to the kitchen and look in my wallet, guess what? The money is still there! I now have forty quid in total, the only caveat being that the two notes have the exact same serial number. Notice how this is a totally zero-sum game. There is only 20 pound for each version of reality, and if one has an extra one, another must be missing one.
But suddenly, I feel bad about robbing my past/alternate self of his 20 pound, and decide to go back again, to four minutes in the past, and give my self back the money. But when I get there, guess what? I cannot do it! Because I am going back to my own version of four minutes ago, in which I never lost the 20 quid. All I can do is create a third timeline, which was visited by a friendly version of future me handing out money, in addition to the second timeline which was visited a by a greedy version of future me taking money. (now a three-way zero-sum game)
To put it another way. If changing things five minutes ago does not change the present, then it also does not change any point between five minutes ago and the present.
Let's take another scenario. I make up a codeword, and then go back to five minutes ago and tell my past self (call him me2) that word, before returning to the present. Once I get back to the present, I can never visit me2 again, because he is no longer in my past. In my past I made up the word just now, but he was told the word five minutes ago by his future self.
So if I go back in time and borrow something (an Infinity Stone for instance) I cannot give it back, ever. Because when I next visit the past, it will be my past, not the slightly funky alternative past I spawned last time by borrowing an Infinity Stone.
Thus, when Hulk promised the Ancient One that he would be right back with her stone, she would have calmly explained that this was not possible, because by merely appearing in her reality he had forked it off from his own, and the version of the past he returned to next time would feature a new her, complete with stone, and with no memory of meeting him.
In other words, if changing the past does not change the present, then the past resets itself every time I travel back in time. In every way.
The problem (from dramatic perspective) is, this makes the past a very boring place. I can go back and kill someone (even myself) a million times, but every time I come back they will be alive and well, totally clueless about the many deaths they have suffered at my hands. It also almost makes the present like a computer game cheat mode, because I can double my resources anytime by just going back one minute and robbing myself!
The movie tried to get around this by making time travel dependent on a finite number of Pym particles, but the exact same logic applies here. You can go back five minutes and rob yourself of all your Pym particles as many times as you like. Sure those timelines are now not going to be able to stop Thanos, but ultimately, what does it matter? There are already, according to this logic of time travel, infinite timelines where Thanos wins, and infinite timelines where Thanos loses, and none of them matter to you at all. Maybe those suckers you robbed in the past are better off without their Pym particles? You will never know or have any way of finding out.
Clearly this logic of time travel, whilst certainly theoretically appealing, creates levels of silliness that no ordinary film could hope to cope with. I needn't bother revisiting the specific instances but suffice to say that the movie was forced to throw its own time travel logic out of the window almost before it had been elaborated by Banner in the early discussions, and continued to throw it out the window to the bitter end(game).
There is one possible answer to this, which the more generous devotees might propose. Perhaps Banner is wrong about the past not affecting the future, but either doesn't know it or doesn't want to tell the others because the implications are too crazy to think about. Sadly this makes no sense on many levels, for instance because Tony Stark would clearly have invented time travel a lot earlier had he been aware of time traveling versions of himself and others before the whole Infinity War happened. This, the version of Stark visited and robbed by his future self certainly was, unless that technologically advanced SHIELD building had no CCTV.
In conclusion, love the time travel concept proposed by this film. It deserves a thorough treatment in a film willing to explore its weird implications. But sadly this movie does not follow it through, and never could have done, given all the mad fights, one-liners and dramatic moments they had to fit in.
In hindsight, it looks like the Russos were desperate, they painted themselves into a corner, and they resorted in the end(game) to the last refuge of the script charlatan - an inconsistent time travel premise - to creep out of it. It was still a fun time. But a quality film or satisfying conclusion to the story arc, not so much.