But again RedHat and Canonical for example can't ship without updating the changed kernel code to match the driver, so can't they leave it to them to fix?

Red Hat doesn't ship any proprietary module. However, they do list a number of symbols and struct definitions as stable, unlike the upstream kernel, and preserve them for the life of a RHEL major release. This is why RHEL stays on an old kernel release and backports new features to the old release, instead of releasing new kernels.

Red Hat and Canonical aren't suckers; if you dump an infinite maintenance burden on them they will think very hard before accepting it.

What are they going to do? Ship a distribution without proper graphics support?

Basically yes.

Note that most of the GPU driver problems are on phones and neither RHEL nor Ubuntu officially support phones at all. And there are also many PC components that RHEL/Ubuntu don't support either, although usually that's just because the hardware is too new.

The wayland login in Ubuntu 18.04 already broke Nvidia's proprietary driver.

So yes.

Red Hat ships nouveau. Nvidia is the one that chooses which distros to support (in addition to upstream kernels) and adapts their driver shim if needed.

Your mistake is thinking RedHat and Canonical are some sort of players here. The vast vast majority of Linux deployments are not from any sort of Linux distribution company.

Nvidia makes way more money from Quadro and Tesla cards than from the consumer cards (probably in general, and certainly running on Linux). "Company-made" Linux (including CentOS) is absolutely a player there.

Disclaimer, I work for Red Hat.