I think a lot of them understand, but from the thread it appears that many gamers bought the PS4 because that console had the best hardware and was therefore the best place to play 3rd party games. That will no longer be the case when the PS4K comes out and as a result they feel cheated and angry.
While it's true that the PS4 will get the same games, and the PS4 is no less powerful than it was before, these things don't exist in a vacuum. There are always comparisons, and it seems many of these people have a problem with perceiving their existing console and/or it's games as inferior or "gimped". Many simply won't be able to enjoy the games as much if they know that the PS4K is able to play them better. A lot of it is mental. Every frame hitch or sub-1080 game will be blamed on devs not optimizing for PS4, etc. To avoid this, they will feel "forced' to upgrade even though the only one forcing them is themselves because they can't deal with having a lesser experience.
Now, someone is thinking, "PC's always better, why don't they just buy a PC" which is true, and some people will do that. But in many console gamer's minds, PC and Consoles are still apples and oranges, whether it's true or not. They don't care if the PC version is better, but they WILL care if PS4K is better than their PS4 and it will hurt their experience unless they fork out the dough for the new one.