For Guerrilla Games, making their own game engine like Decima may have been a risk, but it ended up paying off for them in spades. The developer’s engine powered not just Killzone Shadow Fall and Horizon: Zero Dawn, but will also power Hideo Kojima’s upcoming Death Stranding (albeit in modified form).
The Hideo Kojima connection actually goes deeper than that though. The whole reason the engine is called Decima, for example, is in honor of Guerrilla’s partnership with Kojima. As revealed by technical director Michiel van der Leeuw while speaking with TNW, the engine was formerly unnamed, but after the decision was made to share it with the then freshly onboarded Kojima Peroductions, Guerrilla decided to name it the Decima Engine, after the Dejima Island, a port of trade between Japan and the Dutch in pre-Industrial times (the Netherlands being where Guerrilla themselves are based, of course).
As we already know, sharing the engine with Kojima was a show of good faith, and one that was intended to welcome Kojima to the Sony family.
In the end, of course, Decima Engine stands as evidence of Guerrilla Games’ mastery over their craft—and will presumably go on to power a whole lot of Sony first party games in the coming years.