Something I realised tonight about the final timeline in The Cutie Re-Mark...

In Cloudsdale, let's call Starlight and Princess Twilight 'Present' Starlight and Present Twilight. While Present Twilight always goes back through the time portal to the present, Present Starlight remains. Since she's still there when Twilight returns, we can assume that Present Starlight stays in the past every time and lives out her life for several years until the present day...every time, that is, up until the ashlands timeline.

Therefore, besides Rainbow Dash failing to perform the Sonic Rainboom, the one other key difference is that in every timeline except for the last one, there's two Starlights running around, the one who belongs in that timeline and one from the future who stays in the timeline and attempts to live her life, possibly bringing about her ideals, possibly fighting off some of the big bads that pop up. Every time, someone presumably succeeds in dispelling most but not ALL of the villains that make their return to Equestria, but in the last timeline, there's nothing left except for ash and dead trees. Given Present Starlight Glimmer was removed from participating in this last timeline, I would say that it was her lack of presence that caused this apocalypse relative to any of the other altered timelines.

Thoughts?

That would make sense and it is a very thoughtful way to go about it. It answers the question of Starlight going back to the future the fast way or the long way and why we never see her at all.

I also developed a theory of the Wasteland Future. Due to how bad everything looked, I saw it as an outcome to an event akin to the IDW Comics Godzilla Cataclysm.

In Cataclysm, every single kaiju all around the world were fighting each other, causing devastation everywhere they go until nothing is left standing and the planet is becomes a wasteland with humanity left to pick up the pieces. The kaiju since then vanishes and are seen as myths or gods.

At least that's what I came up with.

4945169

While Present Twilight always goes back through the time portal to the present, Present Starlight remains. Since she's still there when Twilight returns, we can assume that Present Starlight stays in the past every time

I would have said the opposite. Twilight isn't returning to just after she left each time and meeting a Starlight who remained where she was - if she was, the rainboom would already have been prevented by the time she gets there.

4945169 Makes sense I hope we get more information next Season.