Thanos in IW was not a world ending event, in fact, there were still a lot of people left — a lot, billions of them — after the event. Thanos in EG, should he win, could lead to a world ending event due to his desire to unleash vengeance on Earth. Eternals might show up if he won but there is no way to know now. If there is a question why they didn’t show up, the more proper question is toward the end of AOU, which was pretty close to a world ending event. I am quite confident there is a definitive world ending event that prompts them to reveal themselves and interfere in the movie. in any case, in a motion picture universe in which every product build upon, support, supplement each other, it will arrive a point where it doesn’t look to make sense who and who is MIA during this and that. I am not the one who would fixate too much into that inevitable trap to enjoy a film mostly as it is. Just because things are implausible and inconceivable doesn’t mean that they are impossible. The limitation, sometimes, is just our perception.