Large subs like your r/canada example can and do split. r/unitedkingdom has basically been replaced by r/casualuk in size because a substantial part of its userbase was tired of its constant misery and moaning.
> because something like /r/canada can't easily be replaced.
Yeah. As it became more and more clear that /r/canada was modded by reactionary bigots, a lot of people started moving to /r/canadapolitics and /r/onguardforthee.
You still have the issue that r/canada is the default place to go if you're new and want to talk about Canada. To make matters worse, r/canadapolitics isn't advertised on r/canada, so how are new users supposed to find out about it?
Exactly this! You can create new subreddits as and when you like it, but country subs are the issue. You can't just simply create a new sub when the old subreddit is the default a person visits.
This reason to depart the UK predates the existence of Reddit by some centuries.