I find it amazing that Python 3 is almost 10 years old and it's still not been able to replace Python 2 everywhere. I mean, legacy is one thing, but from what I see lots of people still start new projects with python 2.
Mainly because everyone treats it as a different language from python 2.
A better headline would be that the default for newly-created applications will switch.
Existing applications on Heroku will not have their Python version changed.
I assume that's what is meant by "default".
I wonder if you'll still be able to set the Python version explicitly in the runtime.txt file
> Existing applications will not be affected by this change.