Sometimes a game is such a wild success that the sale of new copies can sustain development for years, Minecraft and Kerbal Space Program for example, but even those juggernauts have eventually introduced new versions where that promise was never made. Minecraft Java edition is still entirely free of paid content, but Minecraft Bedrock edition which is the version compatible with consoles and mobile devices has all kinds of stuff. Kerbal Space Program kept their promise that early buyers got everything free, but development on the original has shifted to pure maintenance as they're about to ship Kerbal Space Program 2.
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Personally I've never understood the problem some people have with the concept of DLC as a whole. If the developer can make more content which I can then choose to buy if I feel like it'll be worth the money, that seems like a win-win. Just like expansion packs in the days of boxed games (and still in physical tabletop/board gaming), or buying mission packs to upgrade your shareware copy of Wolfenstein 3D.
There are definitely certain ways that DLC has been abused which should 100% be shunned. The model used by all too many "free-to-play" mobile distraction games where you can pay to speed up or eliminate move timers, anything competitive with a "pay-to-win" aspect, multiplayer games segregating players without the DLC to smaller and smaller queues, etc. All that is bad, but that doesn't mean the entire concept of DLC is bad.
The important thing in my book is that the only reason I should feel the need to buy the DLC is because I want the content the DLC provides. If I feel like I have to buy something to remain competitive with others who have, or to keep being able to play with my friends, or even to just keep playing at all that day then that game is doing it the evil way.