Today, it seems that Vulkan is really the most portable engine available. OpenGL support has kind of stagnated.
I would have thought SDL is probably a better target for this type of application. However, I'm happy to see developers use what they like and push the envelope how they like.
Just upgraded to Linux kernel 5.3 and beta mesa & vulkan so I can drop in my rx5700xt, which I still need to do.
The problem is that vulkan was specifically designed to throw away an amount of portability in favour of allowing lower level access to hardware features. If said hardware features don't exist on a platform, you're stuck. Note the "The Vulkan Portable Subset" having to throw away a number of features to have it work on a number of underlying platforms https://www.khronos.org/vulkan/portability-initiative .
The advantage that opengl has is a number of software implementations that can be used as a last ditch on unsupported systems. Efforts for software vulkan implementations are taking a long time to appear.
To chime in: it's not, but a software renderer wouldn't have the desired performance when rendering at the typical resolutions we see today (ie. up to 4K) - or would require a lot of work to get there.
How is Vulkan more portable than a software renderer? And if you need hardware acceleration, OpenGL is more portable than Vulkan.