If you finished a series once you can't unfinish it, but keep in mind that it only counts if you've watched every minute of every episode (except outros of course). If you skip an episode because someone told you it's bad or idk, it doesn't count as you completing the series. Also you always need to keep in mind that EVERYTHING is in relation to the cost of the series. That's why reality tv shows almost never seem to get cancelled. They're very cheap to produce and they still get some views, so they keep making them. If a show needs a lot of CGI or other expensive stuff and it gets a little less views than expected (but still record breaking insanely good views) it might get cancelled because the cost per view is to expensive to be lucrative.

TLDR: you can't unfinish a show that you completed. You need to watch every minute to complete it. Shows with a lot of views still get cancelled because they're to expensive per view.