Then you are able to use the proprietary driver so long as you agree to its licensing terms.

I suppose that you are arguing that an OSS driver licensed under more permissive terms would encourage GPU manufacturers to contribute more back to the original driver. Maybe it would and maybe it wouldn't. It certainly wouldn't discourage them from not contributing back.

Somehow I think that if they wanted parts of their drivers to be OSS they would have licensed them that way in the first place.