Though Eternals failed to answer any further Infinity Stones mysteries, the Phase 4 movie did suggest why the MCU needed to destroy the Stones after ‘Avengers: Endgame’. In short, the Infinity Stones were always too powerful and their uses so broad in scope that allowing them to remain as key parts of the MCU narrative would have introduced too many loopholes and special powers that would have made real drama impossible to sell. That much is all suggested by Eternals' ending, which sees Gemma Chan's Sersi effectively going Super Sayan to take down the Earth's emerging Celestial Tiamut.
Eternals ends with the Celestials' private bio-robot army realizing the moral issue with their reason for existing and turning on their creator, in another exploration of free will in the MCU. Faced with the abominable task of stopping the MCU's version of a God from crumpling Earth like a smashed eggshell, Sersi takes her role as the Eternals' new leader as the impetus to unify the superheroes' powers through her to turn the Celestial Tiamut to stone. The twist comes courtesy of Phastos' development of the Uni-Mind, which unlocks the potential of all of the ‘Eternals’ to create an almost inconceivable power level capable of killing a cosmically-powered alien God. It largely comes out of nowhere and while it works as a solution - and fits the narrative drama of the "family" realizing their strength as a unit - it does rather obliterate any sense of future threat to them.
Essentially, it's now hard to imagine any threat to the ‘Eternals’ that couldn't be overcome by again using the Uni-Mind to bring every individual power together. -Screen Rant