> What I don't understand is why infix is so important. The only advantage of it I can think of is familiarity
this attitude (that familiarity doesn't matter) is a major problem infecting current Linux userspace development. Gnome, Systemd, Unity, Wayland, (KDE4 to a much lesser degree) are all doing things that assume that familiarity with the existing stuff doesn't matter and that anyone who objects is just a stick-in-the-mud who needs to get with the times.
It's hard to overstate the value of familiarity. Even if the existing tools are inefficient, if they are familiar to people, it can be really bad to change them.
outside of software, one common example is the DORVAK vs QWERTY keyboard debate. Some people argue that DORVAK is significantly better (I'm not one of them by the way), but QWERTY remains dominant, even in things like phone on-screen keyboards, for the simple reason that it's familiar to people.