Instead, this sets the stage for lots of OOPish JS which feels very different from a lot of existing code out there, and will result in awkward shims to interface the different paradigms.
We already see such ugliness in C++.
That's a bit strong. Most languages come with a standard set of data structures, but that doesn't mean they make it impossible to implement other data structures if it suits you (e.g. few languages include binary trees as a standard data type, but that doesn't mean you can't use them).
C includes standard string algorithms, but that doesn't mean you can't write your own — you can even create your own string type if C strings are problematic for some reason.
Many things in many languages are there to offer a reasonable default that is generally useful. This seems along those lines to me.
That has never been the case with most standards. No see why start now. If it's a standard, it should just be present to all engines, and (but not necessarily) the most popular option.
Nothing about it being a standard necessitates it should be "the only option".
>Instead, this sets the stage for lots of OOPish JS which feels very different from a lot of existing code out there
Quite the opposite. This sets the stage for finally having code that looks THE FUCKING SAME as other code.
Instead of 200 ad-hoc implementations of the same exact OOP pattern -- which is what we have now.
Lua is another well know language that has prototype-based inheritance.
`class` just picks one and pretends the rest never existed.