Yes, when they have a choice and resources. It's not even the case here. I.e. it's not like developers simply prefer lock-in when dealing with some of the walled garden platforms. They simply have no choice - the likes of MS, Sony and others don't leave them that choice. It's not a discussion on technical merits. You could use your logic if developers could actually choose.
And if you don't have resources to ask/communicate than just use something like https://github.com/bkaradzic/bgfx/ which is an awesome API agnostic rendering library.
What would be the point for them to use the benefit of cross platform API, if they need to write their own? It won't be any cheaper than using the existing lock-in.
What's big and bulky - shaders themselves, but there are solutions to translate them between different available languages.
Those solutions aren't really ready yet: https://github.com/KhronosGroup/glslang/issues/200
Yes, that's what I meant too. It's not ready (translation to SPIR-V from HLSL). GLSL → SPIR-V should be in better shape.