What if I am not running into any significant Unicode issues for the past 6 years?? (insignificant == stuff I can fix in 10 minutes). What if async/await has been achieved through other libraries (e.g. GEvent, threadpools, whatever)... What is the incentive to switch a large (working) codebase?
A lot of bugs disappear (the API is unified, division is true by default, imports are absolute), or are easier to detect (clear exception if you compare incompatible types, bytes are very explicit).
Many duplicate ways of doing things are removed (modules are renamed, builtins have been cleaned up), the syntax is terser (no more object in class, no more from __future__, super() has a shorthand, no more # coding, not more u'').
It's easier to write lower memory code (most things are generators by default, yield from allow to delegate to sub generator).
Creating clean library is also much easier. Now I'm playing with type hints, and yes, it does help to avoid bug. "raise from" makes error reporting much better. No relative imports make organizing code much easier.
Basically everytime I go back to Python 2, it feels like a pain. So many little things I didn't notice, take for granted, but that adds up quickly.
The worst being of course to code a 2/3 compatible code and to have to do things like: https://github.com/Tygs/ww/blob/master/src/ww/types.py. But I do it only for libs that I want to use specifically to help me migrating project (ww wraps everything the same way in 2 and 3, making migrating easier).
I look forward for 3.6 to introduce f-strings, which I will adopt asap because it's so convenient.