I guess what I'm "doing wrong" is to expect sensible out of the box behaviour. I'm fully aware that I can make it do whatever I want to if I put enough work into it. That holds for almost everything in life. What's limited in my life, though, is time.

Installing Python 3 alongside the old version didn't work for a long time after Python 3 came out. If it works now, that's fine. I compiled my own Python 3 a long time before it finally appeared in my system's package manager.

[Edit] And one more thing. I'm not a sysadmin and my interest in system configuration and change management is very limited. But I've been using Linux for a very long time, so if I'm still doing it wrong then many people will do be doing it wrong and that means there's something wrong with the way it works.