Plus she has onset dementia and senility
To be fair, the only person saying that is Vivienne... who's the leader of the Loyalist (and Chantry Apologist) faction, has a low opinion of Fiona and the Rebellion anyway, presumably is harbouring a lot of resentment towards Fiona for being picked as Grand Enchanter over someone like her, and otherwise, often comes across a snide to those she doesn't like?
Except it's not 'only a matter of time'- time in most respects favors the Mages, because all their immediate concerns (Templar army, imminent eviction) are imaginary. Mages aren't the only one to be blamed for the Divine's death, and there's no indication that the people supporting the mages (Redcliffe and the Monarchs of Ferelden in particular) have any change in their view because of it.
Rationalizing the choice to side with Tevinter because of imminent political fallout is not really that different from rationalizing the choice on the grounds of an imminent Templar army. Both reasons are unsupported by the facts on the ground, paranoid in their perspective, and ultimately non-existent.
Now, calling Tevinter a 'mage positive' country and treating it as any less of a 'slow death' than the Circles, when the mages are marching off to be military slaves, is also a bit weird, but whatevs.
Exactly, the Mages gain nothing by attacking Haven, which would only further harm the already poor image that people have of them?
Launching an assault against the people who just saved Thedas from being swallowed up by a giant hole in the sky, is such a daft move, who would ever willingly agree to it, unless they were being manipulated or controlled in some way?
Even if they feared the Inquisition having a Templar army at their disposal, that army currently consists of recruits, a couple squaddies and a scant few officers, who managed to escape most of the order being wiped out at Therinfal or turned into monsters?
Furthermore, Templars are either disbanded, conscripted or joined as allies of the Inquisition, so at least two of those three basically neutralised them as a threat to the mages for the time being, while the other has them being reigned in by the Inquisition?
So far, taking care of the Breach, closing Rifts and fighting demons is all the Inquistion cares about, not the Mage Rebellion. Given that Rifts are everywhere and demons are far bigger threat than the Mages, the Inquisition has far more work to do than deal with them and realistically, that gives the Mages a lot of time to weigh their options and figure out what to do next?
Launching a suicidal attack only weakens their cause, what would be the point?