Earlier quoted context omitted.
Downvoter here: I downvoted early on because you're making an argument that seems to me both not particularly good and off-topic to the post you replied to. OP posited: “Apple's been taking a number of steps over the last few months to show that they take Safari/WebKit development seriously. This is another positive step in the right direction.” You basically responded, “They're still not as good as I want them to be…
The problem isn't necessarily Apple's laziness with updating Safari, I think it's more the fact that they insist on using their own JavaScript engine. They could very easily just use Blink and V8 which would significantly cut down the maintenance work in keeping Safari up to date as well as leveraging the work already done rather than trying to reinvent the wheel to nobody's benefit. For example most ES2015 features…
Now with Safari Technical Preview, if you want access to all of it in a more accessible package than the WebKit Nightlies, you can have it.
BTW, Apple has been doing what appears to be a amazingly good work on JavaScript Core lately, extending their performance lead: https://webkit.org/blog/5852/introducing-the-b3-jit-compiler....