I agree with your sentiment, and I think what it really comes down to is that Apple and nVidia have been at odds with each other for several years now, and the result is that Apple decided to use AMD products in their machines and have very little care for the customers who want to use nVidia cards. They are marginalizing users of the older, upgradable Mac Pros both to force purchases of newer hardware, and to eliminate the last foothold that nVidia has in the Mac market.
These decisions make sense from a standpoint of selling more hardware and making money, but make no sense at all in terms of cultivating a user base of professional users.
I am not a professional user, so while it ticks me off to have to replace an nVidia card that I purchased less than a year ago with an AMD card for compatibility reasons and to not constantly worry when the next "web driver" will be released, I still placed an order for an RX 560. It still beats the alternatives of overpaying for a machine with no upgrade path or building an unsupported hackintosh that I will have to keep running just to enjoy the benefits of Apple's operating system.
I'm n ot sure there's a rivalry between Nvidia and Apple. I think the AMD partnership for GPU's was out of necessity for custom design work. Nvidia will not do customized SKU's for manufacturers. That means what's in NVIDIA's lineup is what you buy. Apple had their own thermal limits in mind, especially with the "pro" devices that use GPU's. AMD was likely the only one willing to do these custom SKU's for Apple.
it's the similar reason why both the Xbox 1 and PS4 are using AMD chips for CPU and GPU. AMD was willing to do custom Designs. Nvidia is not.
Apple also probably does not want to use CUDA but wants to push to use their own standard. Since AMD supports Open standards such as OpenCL, it's easier for Apple to move forward using a fork of an open standard, than be stuck using a proprietary technology that they have no control over.
What's going on now with the Nvidia drivers not being approved by apple is just probably more of Apple being Apple. Apple doesn't profit in any way from Nvidia devices being used. in fact, since not a single Apple product in 5 years has used Nvidia, Apple probably figures why support it? All it does is help support either Hackintosh's or users who are using their own eGPU solutions that aren't purchased from Apple. They have no monetary / vested interest in helping support Nvidia.