I wonder if we'll see opengl-old implemented on top of vulkan. Actually one thing I would love to see is vulkan on linux accessible from userspace without any involvement of a window server, so that you can write the rest of the APIs on top of it. Maybe even have graphical console running on it.
There's already work to do this sort of thing on top of GL... (eg: glamour the X driver that use GL for all it's drawing and works on anything mesa and the kernel support)
I don't think anything will stop someone from being able to run vulkan directly on to of libdrm like you can right now with GL. At least for open source drivers like Intel's.
Much like the simultaneous ubiquity of C/C++ and Python, in their respective spaces.