At least for Intel, that is just not true. Intel's DPC++ is as open as it gets. It implements a Khronos standard (SYCL), most of the development is happening in public on GitHub, it's permissively licensed, it has a viable backend infrastructure (with implementations for both CUDA and HIP). There's also now a UXL foundation with the goal of creating an "open standard accelerator software ecosystem".
This is all great, but how can we trust this will be supported next year? After Xeon Phi, Omnipath, and a host of other killed projects, Intel is approaching Google levels of mean time to deprecation.
The Intel A770 is currently $230 and 48GB of GDDR6 is only like a hundred bucks, so what people really want is to combine these things and pay $350 for that GPU with 48GB of memory. Heck, even double that price would have people lining up.
Apple will sell you a machine with 48GB of memory for thousands of dollars but plenty of people can't afford that, and even then the GPU is soldered so you can't just put four of them in one machine to get more performance and memory. The top end 40-core M4 GPUs only have performance comparable to a single A770, which is itself not even that fast of a discrete GPU.
At least for Intel, that is just not true. Intel's DPC++ is as open as it gets. It implements a Khronos standard (SYCL), most of the development is happening in public on GitHub, it's permissively licensed, it has a viable backend infrastructure (with implementations for both CUDA and HIP). There's also now a UXL foundation with the goal of creating an "open standard accelerator software ecosystem".