You are making sense.
I see what you are saying. Personally I would not restrict the type of software that should be open source. However I do think your point is valid for what type of software we should make "infrastructure".
Maybe the question is "should we make the codec part of the infrastructure"?
Maybe the browser should not have a built in codec?
Maybe the user should be able to select different competing codecs for use with the browser?
Maybe it is the codec API that needs to be open so that codec producers can write their codecs to an open standard that browsers support?
So the codecs should be either open source or proprietary but maybe *not* tied to the browser?