Plus, we have all the old devices that still work just fine using the old ports. I've been using some LCD monitors for five or six years that are VGA-only, and they are still going strong, with no need or reason to replace them.
DisplayPort vs HDMI is one of the real bafflers. Graphics cards always seem to have one HDMI out, and two or three DPs, yet I have never encountered a monitor that have DP-in ports, just VGA or HDMI.