ABC's Inhumans bit the dust hard, lasting only a season among poor critical and commercial reception – seemingly also scuttling any plans of an Inhumans movie.
But what to do with that piece of the MCU puzzle – a hidden society of heroes with links to the Kree, the Celestials, the Infinity Stones, the Skrulls, perhaps the Asgardians and even Captain Marvel (and potentially even mutants, and maybe the FF)? Enter the Eternals, a similar team of Kirby creations with parallels to the Inhumans in their position in Marvel lore, but with less potential baggage (and more room for a Guardians of the Galaxy-like reinvention on the big screen).
Marvel Studios' biggest brand isn't the Avengers or any one particular character anymore, it's the MCU itself. Think of the Eternals as potentially the MCU's solo origin film.
Given all those factors, it’s possible that Eternals moved into the place once occupied by Marvel’s cancelled Inhumans film, potentially also providing a perfect niche for the Eternals to fill in the MCU’s next (likely Captain Marvel-centric) phase.