> because then they'd have a much smaller and less valuable business, if they even had a business at all.
This does not follow. While GPU drivers are a huge development effort, they are also very specific to the hardware. This is even more true with modern APIs that are closer to how the hardware works.
AMD and Intel both have open source GPU drivers and they are still in business. Are you claiming that their business would be much bigger if not for those open source drivers?