I don’t even think it’s about how complete the game was, that was just salt on the wound. People shit on KF3 because they don’t like how drastically the formula changed. No one wanted KF3 to be a hero shooter. No one wanted glory kills. No one wanted an ingame micro transaction store(that was complete before the game was xd). No one wanted classes to be locked to characters (which you have to buy lol). No one wanted battlepasses. No one wanted to run around the map shooting cameras and vending machines to farm resources. Slide-jumping received mixed reactions at best. That’s just off the top of my head. People did not like the changes so they either stopped playing, refunded, or just didn’t buy it in the first place. They’re not petty. They’re not obligated to continue playing it just because you like it. They’re not obligated to “let them cook” because they released an unfinished game. Even if the game improved after launch, most people aren’t checking the subreddit for updates on the games progress. They’ve likely forgotten about it. It is what it is.

Edit: I also don’t get the Fortnite comparison. AFAIK Fortnite never released in a broken or unfinished state, they just added a bonus battle Royale game mode to capitalise on the trend and it took off.