Charging for updates isn’t intrinsically bad. A good expansion pack at a fair price can be a good deal for both the players and the devs. But there is a modern trend of games trying to squeeze players for every dollar they can get; and when content is deliberately held back in the hopes of selling it for a bit more money later, it starts to become a bit perverse. The game itself can become an advertisement for selling more bits of the game in the future - and it just devalues the experience.
In the specific case of Stardew Valley, the game is a major hit - and it continues to sell well. So even though existing players are getting the new content for free, the developer is still going to get paid. Obviously he could get a lot more if he charged for it, but he has decided he doesn’t need that. He’d rather just make the game as good as he can make it.
Here’s a personal story of mine, about a different game: Several years ago I was selected to be a beta tester for a major game franchise. I was a very well known member of that community, know for making custom balance patches and bugfixes - and so they wanted me to test their new release. I was pretty excited to be a part of that. But when I got my first beta copy, I didn’t really play it much because the game barely worked. It crashed very frequently, and so my feedback was basically just “it crashes when I do this”. I figured it wasn’t worth trying to give balance ideas when the game was in that state. Anyway… time went on, and things didn’t improve much. There were some graphics changes and a bit of UI work… but it was still super unstable. The release date was getting pretty close. But before it was even possible to do a full playthrough without constant reloads to dodge game-ending bugs, there was detailed plans posted on the beta forums talking about the first 4 DLC packs that would be released after the game launched.
I stopped taking AAA games seriously after that. I was totally disillusioned. They were launching their AAA game in non-functional state, with the hope of fixing the worst bugs in a day 1 patch. Very little useful playtesting was done, and so the features of the game were a bit slap-dash, but yet somehow they were dividing up content for as many DLCs packs as possible. They didn’t even have a functional game and yet they were talking about how to sell more stuff. It was a real eye-opening experience for me; and it really colours the way I see other games that launch in a buggy state, where pretty much the only thing that works is the in-game store.
So yeah, I can appreciate the view that maybe charging for updates is a slippy slope that Concerned Ape doesn’t want to step onto; even if he does have very solid footing for if he wanted to tread that ground a bit.