I'm not saying Safari is the new IE6; that would be unfair...to IE6. When it came out, it was world-beating in nearly every regard, but got long in the tooth.
No, Safari is more like IE 7, 8, 9, 10, and 11: predictably disappointing showings thanks to under-funding by an organisation hellbent on holding the web back to defend profits:
hell with it I'll bite
I greatly detest this "the web's potential" and "holds back progress" rhetoric. Every thing ("standard", API, feature, whatever) we add to the web is another thing that'll be abused to hell and back by the sites people use most.
"perception of the web being less-than; a budget option for the unserious"
Ah yes because letting a platform mostly run by unregulated ad-tech and AI firms have access to serial devices, background refresh, etc (most all of which are amazing fingerprinting vectors) is an amazing idea that can't possibly go wrong.
What isn't implemented can be more important than what is (see WebKit's standards positions on their GitHub for some of them), otherwise I fear we're doomed to repeat the BS of FLoC again. For all your talk of the Apple/Android monopoly, the web has sure as shit ended up in one hell of a monopoly.
On that note you focus an awful lot on that profit angle, how iOS abuses its dominance over its marketplace. Replacing one monopoly with another is not the way forward as a lot of companies that missed the whole "hey hardware and software combined is a great cash cow" boat are now the same ones shouting that the "open web" will save it, rather than more logical regulation of app stores.
There's a lot of money on the table when sensible things like ATT, app reviews, etc aren't a problem any more, I find it funny you leave that out.