I am not really fond of the DLC sprout that has become customary in the gaming industry, and especially with Paradox games, but I think your post radiates a scathing cynicism. You are entitled to your opinion of course, and this is the way you feel to express it. Fine.
Being an employee in the IT industry (not in software development though), I can only state that the development cycles Paradox and other gaming companies employ may have something to do with delivering content in an agile way, rather than following a waterfall approach. That is, it is no longer a goal to produce fully polished games at first launch, but to provide for a game that has decent basic gameplay. Then, after launch, it is evaluated if the basic gameplay does what it should be doing. In this case, it is simulating politics in a plausible way for a majority of cases, and not getting it all right to finest details the first time. I say this while being a perfectionist, who actually has difficulties adhering to this in my own job. Afterwards DLCs will be built around features players have expressed to care about and to be willing to pay for. Development goes on from here. Games nowadays remain in constant development, even more so after lauch than before it. Politics may well be a prime candidate for further development. You may think about pricing all you like, and I personally think Paradox may milk their games too much through subsequent DLCs. Your scathing tone suggests Paradox are conn artists, but I do not think that's true. Paradox is actually one of the few companies that keep on developing their games rather than abandoning them within a year after launch. Paradox also tries to listen to its base and to make their games mod-friendly.
I may have spoken before my turn here, as I do not know anything more than you do. My interpretation is just my interpretation of what I have read in the reddit thread and on this forum from Wiz. Screenshots also specify that religious schools provide strength to the devout interest group, which suggests there is one of it per country. But I do sincerely hope that you are right. It would make the interest group system so much more versatile and flexible, and not to mention, more suitable for being further developed in the direction of a realistic representation of political parties as well.