Eh, I think the jury is still out on whether unifying desktop and mobile graphics APIs is really worth it. In practice Vulkan written to take full advantage of desktop GPUs is wildly incompatible with most mobile GPUs, so there's fragmentation between them regardless.
I feel like it's a win by default. I do like to write my own programs every now and then and recently there's been more and more graphics sprinkled into them. Being able to reuse those components and just render onto a target without changing anything else seems to be very useful here. This kind of seamless interoperability between platforms is very desirable in my book. I can't think of a better approach to achieve this than the graphics API itself.
Also there is no inherent thing that blocks extensions by default. I feel like a reasonable core that can optionally do more things similar to CPU extensions (i.e. vector extensions) could be the way to go here.
I definitely disagree here. What matters for mobile is power consumption. Capabilities can be pretty easily implemented...if you disagree, ask Apple. They have seemingly nailed it (with a few unrelated limitations).
Mobile vendors insisting on using closed, proprietary drivers that they refuse to constantly update/stay on top of is the actual issue. If you have a GPU capable of cutting edge graphics, you have to have a top notch driver stack. Nobody gets this right except AMD and NVIDIA (and both have their flaws). Apple doesn't even come close, and they are ahead of everyone else except AMD/NVIDIA. AMD seems to do it the best, NVIDIA, a distant second, Apple 3rd, and everyone else 10th.
If the APIs aren't unified, the engines will be, since VR games will want to work on both standalone headsets and streaming headsets
It's quite useful for things like skia or piet-gpu/vello or the general category of "things that use the GPU that aren't games" (image/video editors, effects pipelines, compute, etc etc etc)