The best argument against continued Canadian involvement in Afghanistan is that Canada’s presence is not doing any lasting good. The Afghan government, in this view, is never going to have the sort of legitimacy with its people that will allow it to function on its own. Canada is simply committing resources, and possibly people’s lives, to propping up a government that will not be able to survive on its own in the foreseeable future.
Peacekeeping and other humanitarian missions are best done when there is a clear goal for them and when they are actually likely to succeed. The Canadian mission in Afghanistan lacks these things. Canadian soldiers are helping to train the Afghan army at this point, but that army is not likely to be able to function without support for a long time to come. The whole point of the mission is to allow the Afghan government to govern the country on its own. However, there is a fundamental lack of support for that government around the country. What Canada is doing, then, is trying to train an army with serious problems so that the army can protect a government with even bigger problems.
From this point of view, Canada should simply pull out. It is not worth spending money and even potentially getting people killed trying to help a government that seems to be beyond help.
The term "peacekeepers" is used all the time to get the public behind military action. I think you may have a vastly different perspective by afghanis and other occupied peoples. We hear about attacks on our forces, but little is mentioned about offensives by the "good guys". The UN has failed time and timea gain while committing war crimes across the globe in the name of peace. The Un is nothing more than a tool elites use for political and military gain.