If the EME are part of the HTML5 standard, but in practice they require some proprietary blob to operate, then the web isn't completely open. It doesn't mean that it's completely closed. Whether that matters depends, I suppose, on whether you want to take a pragmatic or ideological stance.
> More to the point, what do you expect to accomplish here
Does a complaint have to be a call to action?
> If not, I would again suggest finding an argument which will appeal to a non-trivial number of people.
I don't think an argument, as such, will sway many people. If EME (or similar closed technologies) cause enough problems for enough people (for some definition of "enough"), that will change peoples' opinions. Things have to get really, really bad before most people will ask for change.
Was the OS market not open when Flash for Linux didn't exist? FreeBSD? TempleOS?
What percentage of content needs to use EME before you “can't view the web”? If everything else but Netflix works, is the web open or closed?
More to the point, what do you expect to accomplish here – is hyperbole going to convince people to use Linux, cancel their Netflix/Amazon/etc. subscription, etc? If not, I would again suggest finding an argument which will appeal to a non-trivial number of people. Why should they care enough to change their spending or contact their representatives?