Nothing, just false advertising on complexity improvement over OpenGL.
Anyway, middleware has won the battle of 3D graphics, what goes on the bottom layer is largely irrelevant to most devs.
It's not false advertising. Swapping the extension model to explicit opt-in is one of many pieces designed to help you manage complexity in a non trivial project for the reasons I've stated.
So far the amount of boilerplate to handle extension management and code paths in Vulkan samples shows otherwise.
Or, if you care that much, you can not have any of that and just not enable any extensions. Easy peasy
That option hasn't been taken away from you. And unlike OpenGL, you can't unknowingly be relying on an extension since they're opt-in. So what's the problem again?