So, given that these indices could be obtained as needed, I'm not sure I buy this argument anymore.
If we can justify the download of a 20mb for a set of plugins, I think that we can justify a 2mb download to document the entirety of a language's standard library.
As a few points of comparison, Atom's download is around 80MB, Docker 110MB, PyCharm is around 175mb, and a ctags file that covers over 8,000 source files (including boto2/3, aws-sdk-go, the python standard library (both 2.7 and 3.5), the go standard library) is about 6 MB compressed.