Before and After happened where Kes studied her ass off with Tuvok aaaaaaaaaaaaaaaalnight long but she died of old age on Voyager without turning into Dark Phoenix.
I've always assumed that she subliminally learnt too much from her contact with 8472 during Scorpion 1 and Scorpion 2.
If the nano virus didn't work and Voyager was still hip deep in the Borg/8472 war just days later when Kes' abilities bloomed into an ecological shit storm... 1st of all, Kes would be blasting 8472 ships successfully with her mind until they blinked, punked and scarpered. Then second, if Ocampa are the WMD dejour... How long till the Borg have moved through the Kazon picket defending their occupation of Ocampa, craked open Caretakers force field and assimilated the Ocampa entirely?
An hour, possibly 2?