As a platform owner, it is in their best interest to open up their tools. It is always easier to hire developers for such platforms. Even a par game engine from Sony is very attractive IMO. Because developers care more about the size of the audience. The quality of tools itself is secondary.
But we're not talking about CELL processors and VPUs here where only the kings had the keys to the castle. Xbox and PS4 are essentially PCs with their own special secret sauces, it's not hard to make a game for either platform. Easily-available and insanely-affordable development suites like Unity and Unreal are right there to use. Developers have all the tools they need.
I don't find KojiPro developing two separate games particularly feasible any time soon because of the sheer manpower required for a single big-budget AAA game these days. Maybe if/when they can grow to about 300+ people that might be a possibility, unless they have two production teams but one development team who switches over so they're always busy.
Most likely. The way games are made these days, the idea that KojiPro would become like the next PlatinumGames with lots of projects to juggle is sadly wishful thinking.
...That said, if Kojima were to come to Sony and say, "You know, I'm thinking of an iPhone/Android companion app and we think it'd be easiest if we kept the same engine" or "Wow, I got this multiplatform contract bid that'll let me keep staff on after Stranding ships while we go into pre-pro on the sequel, would it be a problem if we used Decima for the prototype?", it'd be something for Sony to think about.
...The other possible side of it is Supermassive, who are tight with Sony now but are still independent. If they were paid to do another Dr Who game for instance, I think they could make an arguable case to Sony that Decima works better in its workflow than Unreal (and they'd have even more experience and feedback to fuel its Sony products.)
It's not like there are secrets in Decima to protect from falling into the wrong hands. It sounds like a great development suite and rendering engine, but the true power behind Horizon is half a decade of hard work and elite coding/designing staff.