South Korea is a small market. The US company is famous for mismanaging its mobile operating systems for more than 2 decades.
The Chinese company didn't "try" to do that, it was forced to. But guess what, in China, where most external apps are banned, there's an entire, completely separate ecosystem.
You do need a combination of skill and critical mass. I don't expect South Korea to achieve this, but for example if there's a bigger alliance of say, South Korea, the EU, etc., where they put resources into encouraging an OS based on open standards, that starts to look feasible.
App developers will need support, but they can port their apps. Especially since this OS doesn't need to start from scratch, it can be based on AOSP.
That's probably the biggest thing Google/Apple are afraid of this point, that a fully open/interoperable mobile OS is somehow enforced. They either have to open up their OSes to be in key markets or they're forced to let someone else create that OS.
