Well, we know from Monsters of the Multiverse that they were willing to split Sea Elves, Shadar-Kai and Eladrin into standalone Races. But they also crested a branching path for some others, like Kobolds. They have different tools available to them.
Yeah.
At the same time, with regard to Mordenkainens elf races, the sea elf, shadar-kai, and eladrin seem noncore and "supplimenting" the Players Handbook races, maybe even monstrous.
Meanwhile, the sea elf has a textbox that actually mentions the inherent "mutable nature" of the elf, and how its mutability will continually cause new kinds of elves to show up in the future.
"That [elf] blood is what causes them to evolve after spending centuries connected to a particular environment, so it is only a matter of time before other kinds of elves emerge."
The elf "evolves" − not by means of DNA − but inherently evolves by means of magic.
Elf evolution is to some degree a choice. The choices of features that adapt well to an environment tend to prevail. The ability to adapt is innate, not necessarily any particular chosen feature.
In other words, elves evolve as a "magical culture" whose individual magical choices tend to be fashionable trends − rather than evolve as a less mutable gene pool.
A core elf race should be highly flexible, to represent this "mutability". The elf player has decision points to build the players own concept of an elf character.
Meanwhile, traditional elf cultures, such as high, wood, and uda, are like convenient pregenerated choices for a traditional elf build. But a player can build their own concept, even when a member of a traditional culture. Players from different ethnic backgrounds can explore their own kinds of elf.
Meanwhile, the ability to mix-and-match cultures, allows character concepts like the child of high and drow parents, or wood and eladrin parents.