Many people use Python 2 for the sole reason it's the default system Python. I assume a lot of them will instantaneously move to Python 3 the moment typing "python" on the command line will invoke the Python 3 REPL.
All my python 2 only code explicitly invokes python 2 on the shebang line.
Re:Release of IPython 5.0
by collinmanderson on Monday July 11, 2016 @09:37AM
(#12070925)
Though, more likely they'll find that typing "python" returns "command not found", and in that case they might find it easier to just type apt-get/dnf install python.
Note - I'm the co-chair for PyDataLondon (and the local conference series). I regularly poll my monthly meetup audience to see how many folk upgrade each month. Normally 1-10 per month stick their hand up, nobody sticks their hand up if I ask "how many downgraded from Python 3 this month?".