Yes, a tyrant could take over, but the only credible threat Canada has had of that since the second world war has been our most powerful ally 'joking' about annexing us. A well-armed militia wouldn't really help to stop that given the difference in equipment and headcount between the Canadian military, and the US military.
If Canadians decide that Canada needs one, we should implement a method to drive enrollment in the the Canadian Army Reserves, or implement a home defense militia that folks could enroll and participate in with a guarantee that they couldn't be deployed outside of Canada (I was in our armed forces, and would not join a volunteer unit that could be deployed overseas, but would immediately sign up for one that was only legally allowed to be deployed domestically for defense).