> Thus OpenCL seems ultimately more proprietary than CUDA,

Could not be further from the truth there! OpenCL is an open standard, can be used to program GPU, CPU and even FPGA. It is definitively NOT proprietary in any sense! To target both AMD and NVidia GPU, you do not need any vendor specific flag at all. Yes, OpenCL, is like OpenGL pretty verbose and explicit. It can be a bit tedious but once you've factored this verbosity in a framework of some kind, it becomes pretty easy to use.

HIP seems interesting, but I am unwilling to invest on it yet. What will happen if for whatever reason AMD abandon it in 6 months? No such worries with OpenCL.