It's true that most of these new features are optional, but that doesn't change that code tends to be written differently in these two languages. I think the relationship between Python 3 and Python 2 is similar to the relationship between C++ and C in that regard.
The goal that all new Python code will eventually be written in Python 3, I don't think will ever be achieved [1]. Personally I'm not using Python much these days, but when I do use it, I still prefer 2.7
[1]: https://semaphoreci.com/blog/2016/11/11/python-versions-used...