The different, separate engine variants for mobile and desktop users, on the other hand, can be based on the same graphics API; they'll just use different features from it in addition to having different algorithms and architecture.
...so you'll have different code paths for desktop and mobile anyway. The same can be achieved with a Vulkan vs VulkanES split which would overlap for maybe 50..70% of the core API, but significantly differ in the rest (like resource binding).
And beyond that if you look at historical trends, mobile is and always has been just "desktop from 5-7 years ago". An API split that makes sense now will stop making sense rather quickly.
And this is the reason why mobile and desktop should be separate graphics APIs. Mobile is holding desktop back not just feature wise, it also fucks up the API.