Refuse? It's not like the Venatori could force them to obey Corypheus. That was the whole point of using time travel to beat the Inquisition to Redcliffe, and then fooling a Fereldan arl into convincing them to sell out to Tevinter.
There's no indication that the mages were in immediate danger. I mean, they obviously felt secure enough at Redcliffe for their leader to come all the way to Val Royeaux to meet the Herald of Andraste.
So what was the impetus for selling their freedom to Tevinter? Because they couldn't win the war anymore? Tough s**t. If you're so eager to fight for freedom, at least have the strength of conviction to stick to your principles.
If they did refuse, there would literally be no conflict in choosing the mages and little reason to ever go with the templars.
Imagine if the game went like this:
Inquisitor: Mages, we need your help to close the breach
Mages: Ok, we join the Inquisition
Mages join inquisition
That's how things would go down. We both know the mages didn't refuse and join the Inquisition immediately was because the writers needed a quest and we wouldn't learn about cory's plans. The same reason why Orsino goes crazy if you side with the mages in DA2 as the writers needed another boss fight.
Also, you're forgetting that Fiona coming to Val Royeaux was pre-time travel, plus venatori agents posing as refugees made the templars seem like worse threats than they were which isn't to say they weren't a threat as templars did attack Redcliffe as the healer says she healed people wounded by attacks by templars in Redcliffe.