Even though Eternals was unable to explain why the MCU would have to destroy the Infinity Stones following the events of Avengers: Endgame, the Phase 4 film did provide some insight into how they came to be.
Before destroying these stones, when it came to incorporating the Infinity Stones into the MCU’s narrative, they were always too powerful and their uses were so wide-ranging that allowing them to remain would have made real drama impossible to sell. All of this is implied by the conclusion of Eternals, in which Gemma Chan’s Sersi effectively goes Super Sayan in order to take down the Earth’s emerging Celestial Tiamut.
It’s fitting that Eternals wraps up with the private bio-robot army of the Celestials realizing the ethical dilemma with their reason for existence and turning on their creator. If Sersi is going to stop MCU’s God from crumpling Earth like an eggshell, she will use her new role as leader of the Eternals to unite her teammates and turn Celestial Tiamut into stone.
The plot thickens thanks to Phastos’ creation of the Uni-Mind. It harnesses the combined power of all Eternals to unleash an almost unimaginable level of power capable of slaying an alien God endowed with cosmic abilities. As a solution, it works and fits the narrative drama of the “family” realizing their strength as a unit. However, it completely erases any sense of future danger for them. It comes out of nowhere.
For the Eternals, it’s now impossible to imagine a threat that can’t be overcome by reactivating the Uni-Mind and bringing together all of their individual powers. Infinity Gauntlet’s concept was based on the unification of powerful individual units that together created something unprecedented, so this power dynamic is nothing new.
Sersi’s display of the Uni-Mind’s advantages serves as a stark reminder that the Infinity Stones had a major drawback. The drawback being that they were simply too powerful for their own good. Like the Uni-Mind, each of the Infinity Stones possesses such a wide range of abilities that nothing could stand in their way. A “catch-all solution” like the Uni-Mind in the hands of either the Avengers or a villain (as Thanos’ idealism actually offered), would be too powerful. There can be no real stakes if there is no limit.
The combined power of the Infinity Stones over death, the usual cosmic power limits, the enhancement of superpowers in individuals, the ability to travel through time and manipulate reality, and many other impressive tricks would be too much in the long run. It would have been a mistake to ignore the Infinity Stone problem in the face of adversaries like Galactus or Kang, who would have been easy prey.
It was the right decision to eliminate them in Avengers: Endgame. Additionally, splitting up the titular heroes for Eternals’ ending was a smart move. This is because it prevented them from simply continuing to use their Uni-Mind against any threat to Earth or themselves. This stays true even if the team was not involved.
There will always be a question of how to limit the powers of Marvel’s newest superheroes as the MCU grows and Marvel Studios introduces more powerful characters.
Phase 4 introduces a whole new Avengers team with the most impressive collection of powers yet, This means that the likes of Vision, Scarlet Witch, Captain Marvel, and Hulk have already been nerfed. This is either through clever narrative reasoning or consciously ignorant writing. It has been done in order to level the playing field. It becomes even more difficult to avoid this problem if artifacts or contrivances like the Infinity Stones or the Uni-Mind are thrown into the mix.