What I don't understand is why we need a 480Kbps interface
to connect a keyboard or even a mouse. My maximum data
transfer rate, as a two-finger typist, is only about 2 bytes per
second, error correction notwithstanding.

The U in USB stands for Universal. It's intended to connect almost
any kind of device.

I'm serious. I find it extremely annoying when I encounter an
application that won't run because it can't handle a USB keyboard
or a USB mouse. IMO there was nothing wrong with the PS/2 or
AT keyboards or mice, or serial mice for that matter (I'm still using
a serial optical Mouse Systems mouse), so I see no reason to
introduce an additional complication. USB is fine for high speed
(or "full" speed) peripherals, but I'm not going to waste a port on a
keyboard.

You've lost me.

A Windows or Mac program knows nothing about the interface used by a mouse
or keyboard. It doesn't even "see" the device! The operating system sends
the program messages with keyboard data or mouse positions.