Hmm.....I have to admit, it does seem to be one of the more reasonable theories.
I wonder where FF:CC's story would fit in with the others?
****MINOR SPOILERS FOR VARIOUS FF GAMES****
I doubt it would be directly after FF9, because while the ending stretch of FF9 shows that the world is covered in Mist (much like how Miasma covers the world in FF:CC), the Mist in FF9 has limits (i.e., it does not exist at higher elevations) and appears to not have much harmful effects to humans that are exposed to it. In addition, we know that the Mist was not the result of a meteor impact, whereas Miasma was borne from a meteor impact.
To me, the meteor impact would more likely put FF:CC either after FF5 (what with the whole crystal shattering and other various things). Now, I have gotten far in FF:CC yet, so I don't know if the Miasma is finally gone by the end of the game, but I suspect that the Miasma streams are coming directly from the meteor that spawned it (i.e., if one were able to travel "upstream" through the Miasma, it could lead them directly to the meteor), and that permanently stopping the miasma could involve somehow disrupting the flow of each of the Miasma streams simultaneously while attempting to seal off the meteor impact zone by using some huge-arse crystals.
In any case, something would need to happen to get rid of it before the events of FF:T or FF6.....that's the only hard part about trying to reconcile it with the rest of the theory.