No, adoption of Python 3 is limited by library support, not install base. Libraries need to be rewritten for Python 3 and some major ones (e.g. Django) are not yet ready. In this case, jQuery 1.9 and 2.0 will have the same API, but 2.0 will not work on older versions of IE in order to optimize for speed. Your comment about Windows XP is apt; this is akin to software dropping support for older OS versions.