Free user space for non-graphics drivers

Even the hw spec doesn't give you security, the hw might not live up to it, see smeltdown.

What an open stack allows you is to fix up the mess once you are aware of a security hole. Worst case you might end up with something like CONFIG_ENABLE_ROOTHOLES to not break existing uapi if the old one is unfixable. But going forward you can change both kernel and userspace driver parts to come up with something to work around the security holes. But often the breakage is really minor (like more gpu hangs in some corner case), so good enough if you just ship bugfixes on both sides. If one part is an opaque blob you can't really reason about, much less change, then that's all impossible. And vendors abandon their closed source stacks real fast, better if they sell you new hw with new drivers.