This article contains spoilers for What If...?
The multiversal chaos of The MCU's multiverse finally became a reality in Loki, when a variant of the trickster god was captured by the Time Variance Authority and placed on trial for the crime of creating a branched timeline. He learned reality naturally orients itself toward a multiverse, with potentially any moment serving as a nexus event that creates branched timelines. The TVA's job had traditionally been to destroy these branches, maintaining the existence of just the one so-called "Sacred Timeline".
Loki eventually managed to pull back the curtain and discover the true power behind the TVA; He Who Remains, a variant of Kang the Conqueror who had triumphed in the last multiversal war. As He Who Remains explained, when branched timelines become aware of one another the outcome is inevitable. Many will be peaceful, but some will not, and these branches will inevitably seek to demonstrate their superiority over one another. He Who Remains ended the last multiversal conflict using Alioth, a predator that consumes reality, and he established the TVA to ensure no new branches were created. His death, and the downfall of the TVA, led to the birth of a new multiverse.
The MCU's Phase 4 has proven He Who Remains to be right. The final two episodes of Marvel's What If...? introduced a version of Ultron who became aware of the multiverse, and who extended his insane mission to destroy all life across the dimensions. He was defeated by the Guardians of the Multiverse, but not before he had cut a destructive swathe across countless different timelines. He was only defeated because the Watcher chose to break his vow of non-interference, forming the Guardians of the Multiverse to serve as his agents against Ultron. This has been followed up by Spider-Man: No Way Home, in which Doctor Strange unwisely tried to help Peter Parker out - and inadvertently broke the multiverse.
Doctor Strange's example proves even the most well-meaning sorcerers can unwittingly break the multiverse. And yet, for all that's the case, it is Ultron who truly demonstrates how dangerous the multiverse is. He was only unleashed because he ascended to a level of cosmic awareness where he sensed the Watcher's gaze upon him. Untold numbers of dimensions were destroyed simply because the Watcher happened to look in on the wrong timeline. It really does seem as though the alternative to a single Sacred Timeline is simply chaos and destruction - inevitably so, because for every timeline where people choose good, there will naturally be an alternative one where they chose evil.
And yet, He Who Remains' solution to this was even more destructive than Ultron. Ultron took advantage of his brief period of time in the Watcher's home, the Panoptichron, to sterilize a huge number of timelines of all life. In contrast, under He Who Remains' leadership the TVA destroyed every branched timeline that came into existence. For every one person killed by Ultron, the TVA erased billions. He Who Remains may have been right in a theoretical sense, but that doesn't mean he was right in of morality, and the multiverse's natural tendency towards chaos will lead to creation as well as destruction. After all, on the basis of What If...?, for every one timeline that becomes predatory there are eight that are not.
