Paradox is practicing a very unique business model by supporting games long after release and without knowledge of the proper business details, any comment I can make is speculative, BUT:
From a players perspective, it is the best case scenario if I get a decent game and the certainty that it will receive proper DLCs for five years or more to come. That is basically the best incarnation of the idea of DLCs, which were promised as a way to constantly grow and improve the games we already love. Paradox is only one of few studious who really even attempt to keep that promise. From PDXs perspective however there is the challenge that every DLC faces a shrinking potential audience, because you are not going to buy the Ancient Relics DLC if you do not own Stellaris already and not every owner of Stellaris is going to buy all the DLC. There are those who skip out on certain DLCs or wait for a while before they go on a sale. On the other hand, DLCs are probably cheaper to make than a entire new game when looking at straight return on investment per Euro, because most features exist and the code base is already there. But these are speculations dependent on many factors and DLCs only work out if the base game is selling well enough.
Now, I want to make it perfectly clear that I believe that the way PDX practices their model is the best case scenario for us players and I would hate to see them fail. When something like Imperator Rome fails to make an impression with the community, PDX can try to pour mmore resources into it to save it, but it may be too late, the reputation be ruined and the project may become a dangerous drain on the studio. If they, however, decided that Emperor was a lost cause and cut it off entirely they would break the pact that they have with the community, which is vital, because it makes us tolerate incomplete, buggy, laggy games, because we love you guys at PDX and respect what you stand for. And half of the fun of your games is hoping and wishing for you guys fixing/implementing stuff.
With all this being said, my conclusion is that the most important thing is the relation between PDX and their community and if they need to raise their initial prices to keep up with player's AAA-expectations... Then just do it. We understand. I have sunken more than 500 hours into Stellaris and people still look at me like the new kid in class. If you raise the price by 5 or 10 Euros, we understand. Or maybe come out with extre premium versions that include extra features that are cheap to make, like twice the amount of character portraits, and let them cost 10€ more. The portraits alone would make it worth for me, and you would have a higher return on investment and the guys that picked the standard version get the same game without missinf out on real features.