While we have previously stated this fact on this very blog, the foregoing bares repeating in light of the subject now discussed. According to the law of conservation of mass, also known as the principle of mass/matter conservation, is that the mass of a closed system will remain constant over time. The mass of an isolated system cannot be changed as a result of processes acting inside the system. A similar statement is that mass cannot be created/destroyed, although it may be rearranged in space, and changed into different types of particles. This is a universal, yes even a multiversal principle.
And so it goes, the Multiverse exists still, despite the seeming permanence left in the wake that was the Crisis on Infinite Earths. But how can this be, since a new composite Earth... known as perhaps "New Crisis" Earth... picked up the remnants of the previous surviving Earths 1, 2, 4, S and X (as well as Earth-8, as it later turned out)? And after another universe-shattering event or two, this world was replaced by first New Earth-0 and now Prime Earth? These are all simply iterations of fractured timelines, and from such divergent realities it is harder to bridge them to the roots buried far below. However, these are mere images that reflect and refract that which came before it.
Definitive proof that these worlds and their respective universes continued on centuries after the Crisis in shown by the tales chronicled of future eras, such as Earth-1's Legion of Super-Heroes, Earth-2's Brane "Batman" Taylor and his sidekick Ricky, Earth-S' Bilbat "Capmarv", and Earth-AD's Kamandi.
Such characters interacted with 20th century characters... who themselves were deeply involved in the Crisis. In the instance of the Legion, they had on multiple occasions traversed time to aid time period wherein their inspiration Superboy had operated, and he along with his cousin Supergirl as well as some of their friends also made the journey to the Legion's 30th century.
For Brane Taylor, on two occasions he utilized time travel technology to pierce the barrier between himself and his own inspiration, the golden age Batman. Similarly, the descendent of Billy Batson, Bilbat, benefited from his own meeting with his heroic ancestor Captain Marvel. In both instances, these more novice new century heroes became all the more so after they gained valuable insight from the fearless forebearers.
Hence, either the Crisis didn't transpire in these timelines, or the outcome was drastically different in that these separate realities did not merged by remained separate and distinct. Thus, we at DC Multiverse persist in our chronicling these Earths as such.