Yes Horizon sells way more but at the same time the market wasnt as big as it is now plus all the social media promotion. Id say Killzone still has juice left, if theres one thing that would make it a way better game than in the past is to get a really good writer. Game can totally do with a reboot even if its just a one-off. KZ2 and 3 actually had great MP, KZ4 looked gorgeous but for me all KZ stories were a bit meh, like an untapped potential because the material and world they created was good but it never made me really care for any character.
If they gonna skip on it, they better come up with something new fast because even though we only have 2 Horizon games, the remasters, lego game, DLC...its already getting a bit boring. Mainly because HZ2 was a slight upgrade to first game. Uncharted 1 and 2 are two worlds apart for example but playing Horizon 2 felt like more of the same. Also unlike Killzone, they shoehorned themselves in this apocaliptic world where the humankind restarted...how many new robo dinosaurs are we gonna get in part 3, its going to be a abandoned planet again, it all feels samey. Killzone had the benefit of futuristic setting where not everything has to revolve around planet Earth, they can always travel to other planets etc, new threat, new galaxy, Helghast planet...