Fair, I’m not saying it would be easy to prove in general (if you have an extortion letter, however, it should be fairly easy), but that’s what courts are for. Also this is not about copying a style or game mechanic, but the whole game. Then again, copyright law differs quite a bit between countries, so overall it depends, but saying this does not involve copyright is just not true.

How will copyright not help here? You don’t have to use the same code or assets for something to be copyright infringement. It’s one thing if you personally don’t believe in copyright, but just claiming that it has nothing to do with copyright because it’s not a 1:1 copy is just not true.

If you made something that “just happens to look like that other thing”, you’ll have a hard time defending that in court (especially if you extort them beforehand). Heck, there have been copyright lawsuits over musicians using similar lyrics or melodies since forever.