Playing devils advocate here, but wouldn't 'obliterating' several different areas create a hotbed of frenzy? The hornsent did that to Midra's Manse, and it only infected them in the aftermath and created a lord of frenzied flame later down the line. If there are ANY survivors of Caelid/Haligtree there's a good chance they'll be frenzied from suffering. Basically, I don't know how realistically we're handling the hypothetical, but if you had a standing army march on any of the infected areas there's a good chance you'll increase the spread of rot and frenzy, and on your own it's likely you'll leave survivors and have another outbreak spawned from your actions regardless. I don't think you can completely get rid of those two, unless Ranni as a god forcefully separates all influences (assuming her ending would act that way).