The APIs might exist, but whether they are superior to the established functionality of a library like jQuery is debatable.
For one thing, these APIs only support selectors as expressive as the browser they're running in, which could be a significant disadvantage for IE8 in particular.
For another thing, querySelectorAll() doesn't actually return the array one might expect, but rather a NodeList. That means that doing obvious things like iterating over all the elements you find with forEach() is, well, not so obvious at all.
So I think I stand by my previous position: a lot of people are going to have good reasons to use another library anyway, in which case they might also have access to various convenience functions, in which case relying on ES5 for the same functionality and therefore relying on an ES5 shim to support older browsers is unnecessary.
