They might’ve giving their immortality to twilight, and probably offered much of their magic to her as well.
Twilight will never accept the death of others, she does not have the right mentality for this - especially not her mentor Celestia, so she will never take the magic from the royal siblings. As a fully grown alicorn, she will likely have understood the internal magic of an alicorn, and thus know what the consequences would be.
Opaline had said that an alicorn is immortal, meaning they cannot die. Then there is only one alternative for the immortals who had their lives extended through magic, ascension from the world of mortals. This is not unknown from Greek and ancient mythology, namely that mortals who gain immortality can join the gods who live separately from the mortal world. And the alicorns are more divine than other creatures - because they are magical beings created in the image of the Ponykind and with a direct link to the very essence of magic. I believe the alicorns’ existence is a result of the ponies’ collective magic and will.
It is likely that the alicorns will ascend and disappear for good when the magic within them demands it, as it has been seen that they did not stop aging and it is known that age is erratic for them, Twilight was once transformed into a filly was she was sick and Luna was able to change her age as seen at the beginning of G4. Celestia and Luna can ascend, possibly transferring their magic to the Sun and Moon who can thus move around the planet unassisted.
It is possible that Twilight may have been trapped in an unknown prison as the limbo when she sealed the magic, this cannot happen with impunity, Fluffy may still be alive in the Crystal Empire whose fate is not known - and Cadence may have gone after Celestia and Luna to the elsewhere, never to return. Opaline has not said anything about the fate of the other alicorns but she pretended to believe that Sunny is the only one who can pose a threat to her.
Discord is heading towards his endgame in IDW #10, Fluttershy’s message may mean that he will follow her in the end, seeing that the Mane 5 can live up to the legacy of the Mane 6 and give him hope for the future. So I don’t think he will play a role in G5. Spike may have become the overlord of the dragons - he is the actual overlord of all the dragons in G4 - and may appear in the future when the Mane 5/New Mane 6 find the dragons’ home.
All indications are that Twilight will appear in G5, but maybe only very few times, the most important will be when Sunny ascends to become a complete alicorn towards the end of G5. Twilight will be there to welcome her to the alicornhood. It could be her endgame.