Speaking of which, assuming that alicorns aren’t immortal, will this mean that Twilight indeed didn’t outlive her friends?
This may not be answered if the writers behind the movie are smart enough not to tell the story of how Twilight and the Mane 6 ended up in the “Elsewhere” dimension.
Thus, we can interpret how it happened, but in my opinion it is possible that Twilight will outlive her friends and continue to rule with Spike and the descendants after the Mane 6, and maybe she will be able to communicate with her friends. Because according to Jim Miller, non-alicorn ponies in the Mane 6 are so crammed with strong magic that they can avoid death - possibly by becoming spirit beings associated with “Elsewhere”, should they end up there. For from the third Kung Fu Panda movie we know that both the living and the dead who had become very strongly spiritually seen (“chi”) are able to enter the spirit realm; and then Master Oogway left the mortal realm - he ascended rather than die to get there. That may be what happens to Applejack, Pinkie, Rainbow, Rarity and Fluttershy after their deaths, and thus may be in contact with the alicorns.
The dimension in which the alicorns occur is obviously as with Discord`s dimension of chaos which is the source of his supernatural powers, important for the ponies as their magic is linked to this dimension, “Elsewhere” where one can look into the past, present and future of every single pony in all generations from G0 to G5. That was what happened when Twilight ascended and gained the aliconhood, her whole life was predestined for this. This can be the “heaven” for all heroes and heroines who could become part of the collective magic of the ponies after their death or ascension.
But that probably means they can never return to the mortal realm even though I’m not sure there (Po killed himself to abduct the villain back to the spirit realm, yet he could return) - and about all the immortal and long-lived alicorns like Celestia and Luna must ascend when the time comes, we have a mechanism that can close them from future generations. Currently it is possible that the alicorns had to ascend when the magic disappeared, to save the balance in the world, not least by making the celestial bodies move around Equus and maintain the background magic.
They will not be gone, but they will not be in the mortal realm - gone for all but perhaps future alicorns.