Of course lightmapping is just one part, but the major focus of the 4.0 release is the entirety of the Vulkan backend. There is a lot more code there than just this.
I don't think your second paragraph is quite true and it's only because Vulkan is already supported on one of the consoles (Switch) and has been promoted by them for years as a way to make it easier for PC developers to port their games. There are also things like MoltenVK (which was a prime motivator for Vulkan support in Godot) that prove that there is some demand AND that other companies are willing to pay the expense of maintaining it, so it's not something that has to cut into their business model or mess with other products either.
I'm actually not sure if Switch has a shippable Vulkan back-end. They've been saying they have Vulkan support for years, but at least 2 years ago there was no Vulkan support to speak of.
Even if it is now finally supported, most of the game developers I know of are using the NVN backend.
I don't think your second paragraph is quite true and it's only because Vulkan is already supported on one of the consoles (Switch) and has been promoted by them for years as a way to make it easier for PC developers to port their games. There are also things like MoltenVK (which was a prime motivator for Vulkan support in Godot) that prove that there is some demand AND that other companies are willing to pay the expense of maintaining it, so it's not something that has to cut into their business model or mess with other products either.