Well, because you're developing against an unsupported runtime. No bug or security updates. If that's fine for you, feel free, otherwise get with the (supported) program.
In addition I don't think you'd have to interrogate every line, depending on your codebase and upstream module support for Python 3. There are tools like:
I would argue I am coding against a stable runtime. No unexpected bugs due to upgrading. No need to worry about upgrading. For many applications, I don't have to worry about security updates. I think for many cases like closed systems that plan to run for years to decades, python 2 is attractive to even new projects.
http://python-future.org/automatic_conversion.html and https://python-modernize.readthedocs.io/en/latest/
Which should get you most of the way there. Look at https://docs.python.org/3/howto/pyporting.html for more info