[cxr]

User agent string parsing (already of dubious merit on its own) by no means belongs in the ECMAScript standard. It would fit right in with the work that the Web platform standards bodies are doing, though. If anything, ECMA-262 itself has already gotten too complicated and needs to be pared down to a smaller, look-we-haven't-completely-lost-our-minds profile. Compare ES5 to anything that came after ES2015.

Even ecosystems that do have developer kits with massive API surface area like you want (such as the ecosystem associated with the other TC39 initiative) had the good sense to define collections of common classes separately, speccing out their implementation as being optional. Then again, there's nothing stopping anyone from doing exactly that and just maintaining it outside the scope of the technical committee, a la Boost or Qt in the world of C++. The fact that people try doing this and fail to retain long-term interest from their short attention span colleagues gives you all the evidence you need for why the irreversible step of transmuting that work into a part of JS's core is a bad idea.