"[The ending] Made worse when you realize there was the fourth option for Happy Ending. Dawn's Advent, the last song, brings the Unsung pony back to normal by making it like they were never Unsung. However, it was part of Firmaments which imprisoned and kept Aria Unsung. This could mean that Cosmic Matriarch left a failsafe, in case Aria was not dead. All Lyra had to do was to make Aria play Dawn's Advent and it would be as if Aria was never Unsung, never creating Unsung realm and never Unsinging any ponies, like Comet Hoof or Lyra. "
Aria playing Dawn Advent and retconning the Unsung, however, would not solve the Cosmic Matriarch's apparent cosmogonal incompatibility with the concept of death. An additional solution to the also apparent inevitability of the story that death must exist in order for life to exist would have to be devised by whoever is still in a position to do so against the retcon, else reality finds itself back in that field where Aria was stillborn-yet-immortal and the Cosmic Matriarch in her grief again began annihilating the universe.
"[The ending] Made worse when you realize there was the fourth option for Happy Ending. Dawn's Advent, the last song, brings the Unsung pony back to normal by making it like they were never Unsung. However, it was part of Firmaments which imprisoned and kept Aria Unsung. This could mean that Cosmic Matriarch left a failsafe, in case Aria was not dead. All Lyra had to do was to make Aria play Dawn's Advent and it would be as if Aria was never Unsung, never creating Unsung realm and never Unsinging any ponies, like Comet Hoof or Lyra. "
Aria playing Dawn Advent and retconning the Unsung, however, would not solve the Cosmic Matriarch's apparent cosmogonal incompatibility with the concept of death. An additional solution to the also apparent inevitability of the story that death must exist in order for life to exist would have to be devised by whoever is still in a position to do so against the retcon, else reality finds itself back in that field where Aria was stillborn-yet-immortal and the Cosmic Matriarch in her grief again began annihilating the universe.