I too find the lack of OpenCL support quite frustrating. It would also be nice to do CPU+GPU hybrid rendering like you can do with Cycles. Houdini (Solaris) Karma renderer will have OpenCL, so SolidAngle really needs to stop resting on their laurels. Because they're so ingrained in pipelines, they've been able to be on cruise control for a long time. The seeds of change are there. Hopefully their grow and motivate Autodesk and SolidAngle to try to innovate once again (maybe just start with making Maya more stable and cleaning up the hot mess of a UI that we've all learned to live with).
I don't think there is any technical reason that SolidAngle couldn't implement OpenCL for GPU and hybrid rendering, but it doesn't seem a priority. They're quite challenged to get Arnold GPU out of beta and provide parity. Given that Arnold is mainly used in larger shops and on big GPU farms, I don't know if they feel strong pressure with GPU rendering, which is more common with smaller shops and freelancers.
I'm quite sure that Nvidia has subsidized a non-trivial amount of their CUDA development. It's smart for Nvidia because it opens doors to sell a lot more GPUs. In the long run, though, I don't think making CUDA and RTX optimized renderers are the right strategy. Nvidia may love it, but it's not good for consumers. AMD's RDNA architecture looks like it will be as disruptive as Ryzen/Threadripper was, and Intel is soon entering the discrete GPU game. A year from now RTX (consumer and Pro) could be noncompetitive, rather than dominant on the high-end.
Speaking of Intel, from my experience their denoiser is better than Nvidia's OptiX, and it will run on any GPU. You should give it a try.
