I think there's a psychological aspect here. if the DLC didn't exist then most people would probably be happy with the game just as it shipped. Then some companies make paid DLC, as a way of extending the game, but also to create tiers e.g. you can pay for a little game, or a lot of game. Now, people are unhappy, and it's not necessarily because they got any more or less "amount of game" per dollar spent.
So, should companies create a "no choices" game where it's one size fits all, or should they create games where there are additional bundles, and you can decide if it's worth your paying for each of the bundles. I think the only people really upset are the completionists who have to have all of the expansions, every single optional vehicle, etc, no matter how tangential each thing is to the core experience. Basically, if no choices exist then life is easier for the completionists, since you don't get upset about not having what doesn't exist.
The additional DLC for Stellaris is additional game material, which cost additional money for Paradox to produce. And the market is limited for that - a subset of people who bought Stellaris will buy each pack, so the price for the content factors the limited market for each pack into it. Creating additional content after the game is launched is a good thing - it means people who bought the core game and finished it can spend a few bucks - only if they want to - and get a fresh experience. The fact that they've created lots of new content for the game you already bought shouldn't be a negative. Would it be a better world if the DLC just didn't exist?
Also, think about this, they could make a $20 DLC pack each 12 months, or they could do the traditional thing, and make a $60 "Stellaris 2" three years on, and basically flip the bird at people still playing Stellaris 1.