The indications that came through the 2021 movie suggest a far much more gruesome past where the G4 abruptly stopped.
Zephyr Heights revealed this. The pegasi city did not appear to be more than a few centuries old and the architecture is inspired by Canterlot despite modern technology and art style. When Zippy took Sunny and Izzy to her research site, it was through a secret entrance somewhere in the city complex - leading down to a station hall. Reminiscent of a railway station hall, but with features taken from the airports with terminal buildings. The glass window was mounted on sliding doors leading to a flat place, we know from the IDW comics and partly the G4 show that an airship port is located in just such locations. The items including the furniture do not show that they were very old, and Zippy, who had probably investigated the place she had found, said that no one could remember this.
Argyle who was far more than what he pretended to be, as it is possible that he knew more than others - had explored “the ancient past”, but his building is full of objects and other things that do not indicate that these are many centuries old. Instead, it seems that these are only several generations old, and magic that could protect them from the ravages of time did not exist. The lighthouse existed when the ponies were united, this was discovered in the station hall. It was not that old, and there was a mysterious mechanism there for the three crystals that Sunny discovered (and led her to the third).
It’s like with the IDW S10 … where the zebras had lost their history, the diamond dogs lost their community and the abyssinians lost their magic. We do not yet know how this happened, only that the Harmony Knights were involved. But we know they were defeated in the end, as the show ended twenty years after that. We saw Canterlot becoming a megacity at the end of the S9, and it is possible that Zephyr Heights was developed in peaceful times as a suburb of Canterlot. But what happened to Canterlot? And it seems that only Argyle who had a map, knew about the city - which Sunny seemed to ignore.
The loss of magic, collective memory loss and loss of community - if this was a hostile attack, it could only have been done by a powerful enemy that even Twilight Sparkle could not stop. And it is possible that she may be alive in the G5, but we do not know yet. When the crystals were activated, it was because Sunny had united the three leaders and talked about love, forgiveness and friendship. She was the hope that made the ponies listen to her. Then it’s no wonder she’s becoming an alicorn, but it’s only halfway.