If this is such an issue and can actually be a performance increase, then someone should release a script with the same functionality. Or optionally make it an opt-in option in settings.
That aside, your position is unrealistic: browsers regularly break non-spec conforming websites. They actually monitor such cases (telemetry) and try to work with popular websites to fix the issue before they ship the breaking update, but it's a tradeoff that is regularly made nonetheless.
Why downvote and no explanation? I want to know if I missed something.
The standard also defines that scripts that are not async have to be executed before later content can be parsed (because they can document.write()). They can still be loaded with any priority the browser wants; they just need to block observable DOM construction.