OneAPI is already cross platform through codeplay’s implementation which also can run on NVIDIA GPUs, its whole point is to be open cross platform framework that targets a wide range of hardware.
Wether it would be successful or not is up in the air but it’s goals are pretty solid.
So basically, a thing that will provide first-class capabilities only on Intel hardware, and won't be really optimised for maximum performance/expose all the underlying capabilities of the hardware elsewhere.
What the point of using OneAPI, a yet another compute API wrapper, to make software just for a single platform?
You can just use regular computing libs, and C, or C++.
Serious HPC will still stay with its own serious HPC stuff, superoptimised C, and fortran code, no matter how labour intensive it is.
So, I see very little point in that.