The Marvels , you know that new superhero movie

[Zalabim]

I get how they got there. It was a writing trap. They needed the Eternals to discover "the truth" of what they were doing. Having them recover repressed memories of past lives on previous planets does this. But, by introducing that into the Eternal's past, it created another problem. One which they didn't seem to notice, much less address in the film.

The dinosaurs died out 65 million years ago. The Eternals mission on Earth lasted ~7000 years. They could have finished over 9,000 planets just since then. The standard cycle from seed to emergence of a celestial is described as eons. That's 1,000,000,000's, billions with a B, years. So while I don't see any reason for the process to be changed if it is cyclical, the Eternals have definitely not been at this for billions of years.

The Eternals are an example of a Celestial creation trying to avoid the problems with the Deviants. They do not evolve or reproduce. They are static and finite, and they still turn out to not do their job or turn on the Celestials directly. So you can't just wave your hands and say Celestials should have fixed it like it's so easy. The Deviants are self-replicating, rapidly-evolving, predator-hunting organisms. You can wish they wouldn't attack some kind of species that doesn't even exist when you create the Deviants, but there is no easy way to describe that objective, and it's impossible to encode that objective permanently into their being.

Now while the Deviants didn't do everything exactly as planned, they can still be viewed as a positive feature by putting pressure on the desired species to grow, adapt, and evolve, as well as providing a way in to the nascent culture for your later insertion of Eternals to further guide their development. The Celestials are already lying to most of the Eternals about the purpose of their mission, killing Deviants to protect humans, so it's no stretch to include a fictionalized story about why they have to kill Deviants in the first place. Plus I'm sure some writer wanted to have a point about how both sides of the conflict are created and used by the Celestials to their own secret ends.

I'm sick of people failing to understand simple science fiction and blaming it on the fiction, instead of their own failing education. You don't have to like the story. Just don't wear your ignorance out by stretching it all over the internet.[EDIT:]I think there has to be a better way of explaining this.