Now that Eternals is out in theaters and available to the public, there's one important point that needs to be pointed out. When talking about a movie universe that has been set up in the way that the MCU has done with it's films, there are going to be some missteps. There are going to be some pretty massive plot holes as well. It turns out that Kevin Feige and company clearly understood this considering how they decided to approach the biggest plot hole in the history of the MCU with the story of Eternals. It's a plot hole that's big enough that Marvel fans were discussing it almost immediately after the plot for the new film was announced.
The story of Eternals is one where the heroes of the movie have been on the planet for millennia. They've been here almost as long as civilization has been on Earth. That was intentional. The point of Ajak and Ikaris and Cersi and the rest being on the planet on the first place is to guide the development of humanity as the race evolves and establishes its obvious dominance over the planet. Anyone paying attention understood almost immediately that the Eternals being around for 5,000 years meant that they were also around when Thanos came to earth and basically decimated the universe. The movie does in fact, explain why such a powerful team of heroes doesn't intervene in the battle that ensued. However, the film also undercuts its own explanation not all that much later in the movie.
Can't The Eternals Fight Aliens?
The explanation given in Eternals as to why the gang didn't go and fight what was a massive alien invasion of earth is because they all had very strict instructions to not get involved in "human" conflicts. It's then shown over and over again that's what meant by that is that they are not to interfere in wars and genocide that take place between different human factions. That particular explanation makes some amount of sense, despite the fact that Arishem and the other Celestials are already messing with human evolution by having the Eternals on earth in the first place.
The rebuttal to this is that the only reason Ajak and the rest came to earth is that they were supposed to defend human beings against the deviants. However, it's sort of unclear why, if they're there to defend human beings against one group of super-strong alien monsters that are quite frankly too strong for humans to handle, then they wouldn't also do the same when another type of alien invasion happens. The Eternals are there to stop anyone from wiping out humanity or doing great damage to humanity. That's explained in greater depth later on in the movie and better explained why exactly it's so important that humans are to be protected. It doesn't make a ton of sense why Thanos isn't seen as posing an even more otherworldly threat to why they are there and the greater plans that are underway, unbeknownst to the rest of the world.
Massive Eternals Spoilers Ahead
For those who have not seen Eternals yet and plan to do so, it might be best to stop here. Big spoilers that underline the massive "Snap" pothole follow.
More than simply inconsistencies when it comes to what alien invaders Ajak and Ikaris and the rest can fight against is the fact that Thanos' invasion undoes the larger plans of the Celestial, Arishem in a pretty massive way. In Eternals it's eventually divulged that the real reason the Celestials want to protect humanity is not because of some sense of duty and care for lesser creatures. It turns out that these god-like creatures only care about humans being fruitful and multiplying. That's because there needs to be a certain population on a planet for a new Celestial to be born. There's a level of energy these creatures need to be able to feed off of in order to emerge.
There's also quite a bit made of the fact that these Celestials need to be born in order to balance out the universe. This is the true purpose of the Eternals and why they are on earth. They are basically the shepherds of the universe in that they make sure these things emerge from the planet, even if that ends up destroying the planet in the process. It's also made very, very clear that several members of the group consider this more than a duty. It's almost a religious reason for existence. Making sure that the earth is ready for the emergence is so important to Ikaris for example, that when the emergence eventually fails, he apparently commits suicide. That level of dedication is very important to this massive plot hole.
A Wrinkle In Arishem's Plans
The question that arises, especially once Arishem's plan is revealed is, wouldn't Thano's plot run counter to this in a very big way? It's important to keep in mind that the Mad Titan's "Snap" wasn't just wiping out half of humanity. It wiped out half of life in the universe. Considering that a major part of the Eternals plot was that these Celestials are also set to emerge from countless other planets and that there is a period of time in which they must emerge in order to keep the balance and keep the universe going, Thanos' plan would be a major problem. A problem big enough that it seems not only conceivable but more than likely that Arishem would alter the "prime directive" of his agents on Earth and have them join the fight.
It's also important to note that at least some of the Eternals knew their real purpose on Earth, even if not everyone in the group did at first. Ajak and Ikaris both knew why they were really there and at least at one point were quite dedicated to the cause. It seems as if one or both of them would have decided it was important to intervene in order to make sure that there was enough life left in the universe to keep the Celestials from emerging. It's also not quite clear as to why Arishem himself wouldn't just come to Earth and flick Thanos into the ether, but there are at least ways to explain that away. It's not clear how long it actually took him to get from wherever he was, to Earth at the end of the film. It's possible he simply wouldn't have gotten there in time. His agents are already right there, seems like they could have joined the fight. Twice, when you consider there was another fight when The Avengers reversed "The Snap."
There are other times events in the MCU that beg the same sort of question. In The Avengers, it seems as if it would have been in the best interest of the Eternals to join Earth's Mightiest Heroes to fight off Loki and the Chitauri when they battled in New York. That might be able to be explained away as not being a planet-changing event, at least not when it comes to population, however. What is clear is that the MCU seems to understand this particular movie caused a rather large plot point. Their attempt to fill it, doesn't quite do the job.