I don't care what others think or why the IP is put to rest (it had 6 games, it was a success and the franchise is pretty popular). I can't convince you to like the game, but I had a blast with it. Like I said in a post above, there just isn't anything like Killzone gunplay. Everything feels heavy, the guns are super satisfying to use, the graphics, sound and art-design are all top-notch. It had a solid class system and well-designed maps and game mods. It's one of my favorite multiplayer franchises. I vastly prefer it over all the popular FPS like Battlefield, CoD and Overwatch. But like I also said, it's just a pretty different game from these. There just isn't anything else like it left. I fell out of the loop with modern multiplayer FPS. The only other franchise games I enjoyed as much last gen was Battlefront 1+2.
As for the single-player, I think the campaign was still pretty great, minus some of the slower levels. But it sure wasn't as good as Killzone 2+3. It also lacked the dirty and gritty look and went a bit too colourful in some places IMHO. It also lacked the "World War 2 in space" atmosphere because it played so far in the future. I'd like to have a new Killzone that goes back to the early times of conflict between ISA and Helghast.