What if Apple provided a better GPU driver, as a non-free kernel blob?

Same as e.g. codecs for MP3 or other such codecs, or non-open-source gpu drivers; it'll be offered as an option to end users in certain distributions. It's not difficult or some paradox, it's an end-user preference on the one hand, and the ideals of a distribution or whatnot on the other.

I have literally no idea – but since it hasn't happened and isn't likely to happen… who cares?

We can guess - the blob wouldn't be in the kernel, and the interface to the blob would reluctantly be added (or not added at all) and the installer would work against particular kernels.

I care. It's kind of strange if you're against undocumented software but still run on undocumented hardware. I'm hoping the providers of the tools I use can see this too.

Hardware doesn't change. The Linux kernel and related userspace move fast. Proprietary kernel drivers are painful for users and developers alike. Nvidia has held back the entire desktop and been a massive PITA for laptops.

I find it hard to believe that Nvidia has held back the entire desktop, when >50% of computers don't use Nvidia GPUs.