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.

On the contrary, you're acting like you desperately want to be a sysadmin by demanding to control which specific version of each package is installed.

No, I want developers to be in control of dependency management.