> HEVC is pretty amazing though, and it's clearly better than VP9, so it looks like the proprietary stuff beats open source in this field

This is just a restatement of the fact that codec patents exist.

When the state of the art advances because of improvements A to Z and A to W are in the public domain, the proprietary codecs are still "better" because they use A to W plus X, Y and Z which the open source codecs can't use because of the patents.

The real question is whether the patented things would have been invented without the patents. But the answer is probably yes, because the industry is full of huge companies like Google and Netflix that directly benefit from improving compression regardless of whether they can sell it to anyone else.