>>37626336There's a lot at play here. For the sake of worldbuilding, we can throw out theories like "hasbro doesn't care" because that's boring and doomposting.
An easy theory to lean on is that G5 is from an alternate universe timeline. Glimmerfags can feel safe that it's not one of the universes Glimmer created with time travel, since those universes hinged on the mane six never getting together and becoming the elements of harmony. The legends of the elements, whether alt universe or G4 canon, seem to focus on when the elements were younger, hence Twi still being an alicorn but not growing into the full alicorn body yet.
Some time after the finale, it's clear a tragedy of some sort took place. Another pony took up Cozy's mantle and created a magic siphon, the villain trio were released and Tirek stole magic again, or something completely new went about sowing discord and distrust while siphoning magic from Equestria. This could even be a timeline split where the elements and other heroes were defeated during the S9 finale rather than triumphing.
The land gets ravaged by chaos and windigoes for some time. The windigo threat ceases to exist once the magic siphon/Tirek//unknown magic sapping device X takes even their ethereal forms. The villainous force dies out at some point, but not before ravaging nearly all magic from the lands. This leaves Equestria in turmoil. Creatures flee back to their homelands to ride out the storm. The alicorns are defeated in some manner, possibly killed. The pony races, reverting to old tribalism, take shelter in the remaining population centers.
Canterlot is abandoned, and the now-flightless pegasi who hadn't fallen to their deaths take refuge there. Unicorns hide out in what seems like the Everfree forest, made safe by magic no longer affecting flora and fauna, but with a frightening enough exterior to dissuade explorers. Earth ponies of course take up residence in Maretime bay.
cont.