I'm not sold on the "We didn't intervene with Thanos because we were instructed not to get involved except with Deviants" thing. I mean, it's half the life in the universe disappearing. That's important enough to make an exception for.
Maybe it's a matter of them being stuck in their ways for 7000 years?
"Half of all life is gone!!"
"Did a Deviant do it?"
"No"
"Not our job"
Perhaps the first half of the movie is about one of Eternals who has spend to last 1000 years or so trying to get the others to get involved in others things and always get brushed of. And now, finally, slowly, they're trying to see it his/her way. I don't know.