I would not be too worried about the future of CK2. Holy Fury sold well from what i understand and their whole business strategy is more DLC sales = more DLC.
If you want more reassurance look at their language in the Iron Century announcement post last week, here is an interesting line used by Tuscany 'and our first in the 10th century'. Firstly i don't think this means they are planing another 10th Century start any time soon but the open language does not sound like the language of a game planning to stop development any time soon.
While I also originally thought that more DLC sales = more DLC with the release of the new free patches I have noticed something. The retention time of players increases dramatically when free DLC is released. Player wise every DLC except for Holy Fury only gives a boost of players for about a month. Holy Fury only lasted 2-4 months but the small DLC worthy content that they have continously added in small amounts every month keeps the retention time up meaning that the overall playerbase is increasing. While this does give a small temporary malus to sales it gears them up for the long term which Ebba Ljungerud has stated in many of the quarterly reports is the goal of Paradox at the moment. I also believe more DLC is coming because there is no reason to try and up retention time without something coming, even if a new player (or returning player who hasn't played in a while) buys a few DLC it won't equal itself out unless they buy at least 3-4 (on sale) and that is not a given. However if a large DLC is coming that they want a larger player base to help spike it (If they create another equivalent of Holy Fury they could keep >20k people daily for a while).