This is a good proposal. While some will be inconvenienced, the result will benefit the huge majority. Resources allocated to maintaining the older themes can be allocated to more productive efforts. I have used most of the default themes starting with twenty-ten, looking to them as an effort to provide a sound, best in class basis for building a site. I suggest adding information to the “read me” file, perhaps under a heading of “Support Policy” or something similar which describes how this will work going forward.