Two problems with this. There is a very long tail of libraries on 2.x only that are not moving to 3. Second problem is that many 3.x only libraries are not widely used or tested and you'll find obvious errors that would not be there if they were widely used. And as you said, some popular libs are 2/3 compatible.

So what should an end user do? Build on 3, which still has many things missing and less reliable 3.x libraries? Or stick with 2 which has everything the 3-only ecosystem has and much more. 3.x in reality makes no sense unless you have a political bone to pick. The big attraction was the unicode by default.

Python has supported unicode since 2.6. Python3 presents a new default way of handling it, nothing more. Py3 is pure technical churn at its worst. There's no innovation there. As a result the users outside of a vocal minority aren't coming.

When I started my current project two and half years ago I stuck with Python 2 because I was concerned about library support even though the major libraries I was using (Pyramid and SQLAlchemy) already supported Python 3.

While I think that was the right decision at the time, for me the balance shifted about 6 months. Needing access to the much improved multiprocessing library in Python 3 I ported my application to polyglot with the help of the python-future library. It really wasn't very difficult.

This was a modern codebase of about 20K LOC with reasonably high test coverage. For much larger or older applications then not porting is probably the right choice. But for new applications I think Python 3 is a fairly good bet now. Even if only 15-20% of the Python ecosystem have switched then that's still a significant enough userbase for most libraries to have had their Python 3 bugs shaken out.