Viewer assumption =/= a set of rules set by the makers of the show Arrowverse shows.
At no point has there ever been a hard set of rules given other than different versions of Earth exist.
Once again: I never said it was an unbreakable rule, nor am I asserting that they cannot or should not change it. I am merely
commenting on the fact that they have followed a pattern up to this point that they are no longer following. Commenting on a thing does not require taking an ideological stand on it. Observation is not judgment. First one has to accurately describe reality before one can formulate an opinion one way or the other.
In science and reason, you base your model only on the available evidence, and freely amend or abandon that model when new evidence contradicts it. It is certainly wrong to assume that your current model is inviolable for all time and that any further evidence must be rejected -- but that does not mean it is an "assumption" to formulate a model in the first place. It's an extrapolation from the data, a working guideline to use as long as it remains useful, i.e. until new data conflicts with it and requires it to be changed. As long as you don't mistake it for an inviolable article of faith, as long as you change it as needed to fit the facts rather than the reverse, then you're fine.
So the statement that all onscreen doppelgangers up to this point have been identical to their counterparts is merely a correct description of the data. Back when 100% of doppelgangers were identical, it was reasonable to use the model "doppelgangers are identical and contemporaneous with their counterparts (as far as we know)." Last year, when
Elseworlds canonized the 1990
The Flash as Earth-90, this implicitly established the existence of at least a few non-identical, non-contemporaneous doppelgangers, but they were rare exceptions to the overall pattern, so the overall model still held with minor modification ("
most doppelgangers are identical and contemporaneous"). Now, however, the data from
Crisis on Infinite Earths require a major change to the model -- doppelgangers are
often non-identical and non-contemporaneous, possibly even more often than not. Acknowledging that we need to adjust our model is not complaining or protesting or denying -- just the opposite.
Certainly I was happier when all doppelgangers were identical and there was a consistent pattern being followed. It's harder to justify there being a parallel Superman or Flash or whatever living a generation earlier or later than another. But data always outweigh hopes and preferences, so I adjust. I'm disappointed, yes, but I don't deny the new data.
It seems reasonable to conjecture that the 53-Earth multiverse established in
Crisis on Earth-X is a "local" multiverse of closely parallel Earths that do have identical and contemporaneous doppelgangers as a rule, and that there's a wider multiverse beyond that cluster where the parallels are more indirect. Although that does make Earth-90 an anomaly, because it has at least a half-dozen identical doppelgangers or nearly so (Tina, Julio, Bellows, Trickster, Prank, and Barry as Jay/Henry's doppelganger) along with at least a few non-identical ones (Nora Allen, Iris, Scudder, etc.). Similarly, Sherloque Wells came from Earth-221, and he and Renee Adler have identical doppelgangers on dozens of Earths, including Earth-1.
Of course, the numbering of the Earths has been presented very inconsistently, and it's unclear just who decided on the numbers. So their numbering doesn't necessarily reflect anything about their relative proximity. And of course there's no real way to make sense of a multiverse incorporating different fictional iterations that were never intended to have any in-universe relationship to begin with.
