I'm gonna borrow a bit from Dean the Young's argument here
The mage rebellion also doubles as a sort of referendum on whether mages can be trusted to look after themselves outside of Chantry support while under pressure. The fact that they turned to the living embodiment of why the Circles exist for support proves that they cannot be trusted to look after themselves with no oversight.
"But they were desperate"
Then they shouldn't have gone to Tevinter, appealing to the local monarchy would have been better suited, Ferelden may be weakened but I'm sure they can hold off fractious branches of the templars. Instead they betrayed that trust and joined up with the sworn enemy of all southern Thedas
Honestly, what could they possibly have been thinking?
First of all, it is very unlikely the Templars were about to attack given how we are told they were all summoned to Val-Royeaux.
However, given the fact that summon came from Envy, it's possible that they were recalled precisely because the mages could now be manipulated by Alexius.
So, let's go with "The Templars were going to attack."
Ok, if they are about to attack a Ferelden city to reach people under Ferelden protection, it is the job of the Ferelden army to fight them.
This is the opportunity to show Fereldans how the Templars don't care who stands between them and the mages and that the mages will fight to defend the Redcliff and its people after they graciously granted them refuge.
Instead, they invite Tevinter, proving the Templars were right all along and forcing Alistair to march with the army, yes, but to kick the mages out when he could have been marching to deal with their problema, the Templars.
Does Fiona have cheese for brains or something?