He actually did that in a novel to assassinate moira when she did her coupe in Ironforge. SI:7 are also a huge deal in the Alliance questing experience in the last 2 expansions and supposedly they have been around for alot longer. Varian was a warrior but he knew how the game is played. But I doubt Varian ever considered the Forsaken to be "inciteable", he had no reason to assume that there are "good" ones, frankly neither did we until the end of Legion.
Also if Varian knew about Taelia he was quite the dick, since during the cataclysm the status of Kul Tiras was anything but certain. One would think he'd have tried to bring her back into the alliance. But Blizzard has alot of filling out to do with all these gaping holes in the story, I hope we will get some decent answers once Taelia and Bolvar meet.