Who or what do you think was responsible for the final, lifeless alternate timeline created by Starlight's meddling with time?

7102652
I always figured one big battle royal between all of the villains.

7102652
The ponies themselves.
They didn't fix pollution and focus on the environment.

After failing the test to enters Celestia's School for Gifted Unicorns, Twilight became single-mindedly obsessed with proving she wasn't a failure. She studied increasingly advanced magic, but was never ever to effectively use any of it. Eventually she became desperate enough to delve into dark magic, where her studies led her to the Alicorn Amulet. The amulet's power brought Twilight's innate power to the surface and quickly corrupted her. She soon lost sight of her original goal and set out to become even more powerful than even the princess. She comes across the book about the elements and goes the Castle of the Two Sisters to get them, but after seeing them as useless stones, searches the rest of the castle and comes across the book from Inspiration Manifestation. With it's power to make what she imagined real, she finally set out to prove her power to Celestia. Celestia, being as good hearted as she is, tries to reason with her little pony, but Twilight has become blind to all but gaining more power. She defeats Celestia and imprisons her, performing cruel experiments to try and replicate the power of an alicorn.

That's all I got, corrupted Twilight's search for power would eventually lead to the annihilation of all life, maybe she's the last pony alive, ruling a kingdom of nothing from her throne of dust.

I could guess that is the timeline that Starlight managed to win... If they go by order, each timeline Twilight visited was of one villain that managed to win... With Chrysalis, Nightmare Moon, Tirek, Sombra and Discord already appeared, and we leave out Sunset Shimmer, then the next is Starlight Glimmer.

Sure, it could also be a spoiler of Cozy Glow victory but the hypothesis is that Starlight managed her 'Equality' to the point of making even the Princesses equal, and without their power, no one to move the sun and the moon.

7104253
If the princesses lost the ability to move the sun and moon, they would just have to work with the unicorns to move them the old fashioned way. The princesses haven't always been in charge of the sun and moon after all.

7104361
You forgot the power of 'Equality' Remember Twilight when she was under the equal cutie Mark? Just the same for Celestia and Luna, time ago, unicorns could be as powerful as Twilight to move the sun and moon and we don't know how many did they need to replace Celestia and Luna. But in this case, the unicorns got weakened by the equal cutie mark, after Celestia and Luna got marked, they also got weakened.

Let's put it in another way, you need 100 workers of great strength 20 experts in using different tools and create the concrete for example and some good supervisors to build a mansion. What could happen if those 100 workers and 20 experts are replaced by 5 year old kids and there is only one supervisor that don't know anything architecture, building or something related and can't use the tools?

7104371
Good point, it's a little difficult to tell how many equalized unicorns would be required to move the sun and moon, and since the total unicorn population is unknown, they might not have enough to last very long, especially since no unicorn has practiced the necessary magic since Celestia took over.