I was a staunch Python 3 supporter until I encountered such a thing. I avoid Python 3 now because I don't want to be forced to make a decision between writing the requisite library myself (a huge undertaking), abandoning the project entirely, or rewriting everything into Python 2 again. Better to just start with Python 2 in the first place.
If you're using any language other than C you'll always occasionally find yourself in the situation of "there's this useful library that only exists in C". But for many languages the library ecosystem is large enough, and the language better by C than enough, for the tradeoff to be worth it.
Also that's kind of what this post is about. Pretty soon there'll be a bigger risk of finding that some library you want is Python3-only than finding that some library you want is Python2-only.
Even when writing C you still occasionally find yourself in the situation of "there's this useful library that only exists in Java". There's no language that can directly use every single specialized niche library written by a ___domain expert.
