Bit of a late response, but...
There was a point in the history of game development where this 'plain logic' you describe was a foreign concept. Post-release content updates were exciting and rare, and seen as the developer going above and beyond their expected duties.
I think you underestimate the ability of larger studios to treat their games like capital. Development costs are ridiculous for AAA titles, and these days the largest developers (not sure if Paradox counts) take into account the longevity of a games profitability when they decide how much to invest. It's not a cynical attitude in and of itself; these developers don't see anything wrong with this style of development. To say this kind of development doesn't exist is untrue.
I'm not saying that strung-out development is necessarily a bad thing, though it often is. I just can't believe we've reached the point where people are defending it as normal. Times have certainly changed.
The fact many modern games have a post-launch DLC plan doesn't necessarily mean the core game's willfully underdeveloped just so that said plan can exist. Of course companies want their games to be profitable over a longer timespan. This is business, after all. But they also need the money from DLC sales to actually produce said additional content.
While such a methodology can certainly be abused, I generally defend it because it's a positive factor more often then not. Over time, it gives us more fleshed out games than we've had in the past. Does it cost gamers more cash, over time? Yeah, but some gamers want to have the cake and it it too. I'll gladly pay more money to a dev willing to keep expanding a game I enjoy.
There's no rose-tinted past in which game development was better. Earlier games were less complex and therefore less prone to a bug-ridden release. And those that had one were in many cases forever crippled due to the lack of post-launch support. As far as expanded content's concerned, a successful game may have gotten one or two expansions at best. Expansions themselves were quite uncommon.
CK2 was a totally different release than Stellaris or Imperator. Everybody was super hyped for it, there was no real concern of it being dumbed down (which is often the general thrust of the harshest criticism for sequels) and from pre-release to the leaked version through the proper release, people were enthusiastic about it.
And it can't be a matter just of time period preference, or else there wouldn't have been such a big difference in the relative players of CK1 vs EU3 and CK2 vs EU3.
Also, Sengoku and March of the Eagles were never really mainline games, they were meant to be one-off titles. Stellaris may very well have originally been too, they said something to the effect that they were caught off guard by the enthusiasm.
I'm surprised about the enthusiasm around Stellaris, but I suppose much of it comes from the fact space opera sci-fi caters to a large audience. And as it was said earlier there's not much in the way of competition. I'm miffed the DLC continuously expand the later portions of the game, and ignore the fact the mid-game is a gaping chasm.