>As for the remaining millions, only a tiny minority of them will even know this is a thing, let alone care enough to make the change or contact a developer who can do it. These are the folks who have hosted their wordpress site with GoDaddy because it was cheap and quick when they needed a site.
One company decides to do something stupid and you expect millions of website owners to scurry and add junk to their headers to create a "mitigation"? This is nuts.
This is a browser problem, not a website problem.