It probably comes down to a test of wills in the end.
Unless a Templar is focused entirely on denying magic that the mage they are facing is trying to summon, any slip in concentration from the Templar can allow the mage the opening with which they can exploit and attack with.
Furthermore, mage abilities are reliant on mana, which replenishes naturally (albeit slowly) over time, although they do often use lyrium potions to augment their power or speed up mana recovery. Templars on the other hand can only count of lyrium to maintain their abilities and need to chug pints of it constantly to keep their anti-magic at full strength.
The point being that if a mage was able to out of range of both weapons and Templar abilities, as well as hold back enough power in reserve during a sustained fight, then they would eventually be able to overpower them through sheer attrition. That's probably the reason why Templars seem to be trained so that dispelling magic is always their opening movie, seeking to end the battle before it begins, because their odds go down if it turns into a lengthy engagement.