Consider that different users have different needs, and almost all features listed are opt-in.
It means my browser is more bigger and more complex. It broadens the attack surface exposed by my browser, perhaps even if I don't opt-in. It dilutes the efforts of the Firefox team. It introduces new ways for browsers to be subtly incompatible. It further raises the barrier to someone making a serious browser from scratch.
Also, are we guaranteed that these features will be opt-in? There were serious security issues with WebGL (predictably), and I don't think WebGL was opt-in.
hm I wonder if there is a chrome extension for that
This is an issue with that particular website's implementation, not something inherent to the browser or the API.
When that statement applies to 90% of pages out there, that argument gets more stale than cracker left out for the better part of a year.
That staleness, in fact, is why no one is encouraged to run blind unauthenticated proxies on the Internet anymore. Completely valid technology. Very problematic use case.
because the product manager thought my user session was engaging with the site for 2 seconds longer so it must be good.
I never said it was the browser or the API just acknowledging that it is a predictable gripe and not a feature, and yes enabled by the browser and APIs