On saviors of Equestria, alternate

For villains other than Nightmare Moon to win in alternate timelines sans the Mane 6 being Elemental Harmonizers…

The idea of millions of possibilities, though? That requires Butterfly (Fluttershy?) Effect results from this being sufficient to divert away from all of the known powerful threats.

…which leads my thinking to the true answer: to get past Each villain, Equestria must be saved. So there is always somepony who saves Equestria, except in NMM's timeline; it's just that nopony else (or sets of ponies) will be properly situated and able to defeat all of them, a much weaker premise. (Sombra in the Crystal Empire in particular is on a pretty short timescale for "get sent up by train before the royals' shield runs out", as the crystal ponies with any fight have been pre-defeated…and, given it's more or less a state secret, requires the trust of Celestia and then the door requires dark magic. This combination seems like it's going to be virtually nil, save the Princesses themselves. I expect some pegasi see it from outside in some other nonSombra timelines.)

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¹Discord could get let out early by some other chaotic episode, now I think it, and Tirek taking advantage of the missing Cerberus depends on yet other factors…I guess Sombra coming back *might* be related to something other than a thousand years passing, but the newfangled railroad going up there suggests otherwise.

This suggests a few things.
❧Nightmare Moon has the statue garden sealed off, runs no Grand Catastrophe Galas, or moves Discord somewhere actually secure and unexposed to chaos.
❧Discord and NMM are defeatable without the EoH (or Celestia uses them solo).
❧❧Solo use of EoH rather than passing to new bearers means Discord's seal isn't weakened by bearer-switching (seen this suggested somewhere, can't recall at the moment)