So he's capable enough to add new islands and content, but not change a trigger on how to save. One button. Same logic....

Correct. The entire game state was not designed to be serialized. You can see this with how buggy multiplayer is. He could do saving mid-day, but I don't think most players would want that over an entire new island worth of content, especially when the amount of work is comparable.

A buggy mod by someone who didn't write it doesn't mean it's not possible.

I didn't say it's not possible. Anything's possible when it's your code. I said it would be hard to add now.

You can like a game and someone and still be able to be critical of a game design decision they make. Not everyone is good or bad. A hero or villain. No one is perfect.

This part, I don't get. I maintained a mod with thousands of endorsements and over a hundred thousand unique downloads. I talked with modders even bigger than me regularly. Even the lead dev of SMAPI, who has some more insight into the codebase than anyone else in the modding community.

None of us succeeded in it.

At no point did I defend the game on its merits of being "good or bad". I haven't played it in years. All I did was explain that this is technically very difficult to do.

On the topic of dark patterns, many games have specific points where you can save. This is not unique to SDV. For example, some games only allow you to save at specific points to avoid save scumming and increase the difficulty.

What matters isn't when you can save. It's whether your inability to save at times is being exploited for their benefit.