If the entire Inquisition and Ferelden army couldn't stop the Venatori in the bad future, what makes you think Templars are all you need?
That probably had more to do with demons and nobody being able to seal the rifts (no 'Herald) than anything else.
Don't forget how your advisors outright say that even if you could invade Redcliffe, the Fereldens would just see the Inquisition as Orlesians starting a war and would start a war with the Inquisition. Don't forget how your advisors outright say that the Venatori would already be long gone by the time you are done recruiting the Templars. In other words, are you really going to waste time hoping for an army that you know will anger Ferelden and start a war with Ferelden while also allowing a hostile foreign power to take the mages and go to war most likely against you while you are busy hoping not all the Templars are douchebags like the ones in Val Royeau.
Again, I have no interest in invading Redcliffe. That is a fool strategy, which is not only what you're doing in that quest, but you are also going in without an army behind you (just spec-ops)!
I would rather draw them out and fight on homefield or neutral ground. So, let them be long gone.
And I do not have to hope anything. I know it to be true that there are stand-up individuals in the Templar Order, from past experience (which you should also recognize, seeing as people similarly stereotype your poor mages). In fact, I do not need the Templars to be good people either, just willing to help my cause. Given what they stand for, it is very likely one can garner some support from them in a mutual goal of sealing the Breach.
Also, you have no idea Red Templars even exist when you are deciding a mission. All you know that the Templars are hiding out at Therinfal. There is absolutely zero indication your character thinks anything is wrong with the Templars until the Red Templars attack. The entire start of the mission was you essentially discussing the situation with Lucius and hoping some templars will aid the inquisition.
Something called "The Inquisition" should not be so in-the-dark about that kind of a development. Leliana should be taken to task for that. The Inquisition as a whole deserves blame for letting that happen under their nose, especially since what Lucius does at Val Royeaux should have set off some warning-bells.
Also, as for a bigger threat, I'd say the Venatori are a bigger one. When it comes right down to it, the Red Templars are just foot soldiers. They can't blend in and only serve as soldiers so that's all they can do. The Venatori meanwhile developed time magic, managed to trick the Grey Wardens into unknowingly serving Corypheus, can successfully kill Celene, and they have a spy network that can firebomb Denerim, brainwash the Nevarran king, plant a spy in Skyhold, send red lyrium to Tevinter, and take control of a town. Really, the Venatori are a bigger threat considering the Red Templars can only fight and not do anything else whereas the Venatori have way more resources and are capable of subterfuge that can do more damage than the red templars ever could.
Let's remember the bolded for the next time you try to claim that preaching the dangers of magic is just fear-mongering. By your own admission, you admit magic makes people more dangerous than simple foot soldiers.
And what good did going to Redcliffe do since the Venatori get up to all these things anyway? At least you have an actual fighting force within your Inquisition to face them if you recruit the Templars. The mages, OTOH, are not necessarily formal combatants.
Red Templars also get up to some very dangerous things, too, if you would like me to list them.
Personally, I (as the Inquisitor) find Red Templars tougher to kill than Venatori. Ergo, I see them as the bigger threat.