I asked myself this question several times, too, when reading through the docs of python. Many nice things are available with python 3.4+ such as asyncio on Windows and so on.
I didn't investigate it so please apologize the stupid question: What changes break which bigger packages? I had to struggle with some library renames from one to another version, right now, but all of these changes could easily be fixed by an intelligent import strategy.
Maybe some kind of knowledge database could be published to give plugin devs the chance to prepare for the changes in a long term.