KVM switches used to be a huge and super important (and expensive) thing in rack server farms back when every rack or every few racks had a pull-out tray with a keyboard and a monitor. Technology moved on. First the tray and monitor disappeared with KVM-over-IP, which saved many trips to the server room. Then the KVM switches themselves disappeared, as manufacturers integrated out-of-band remote access into the servers, first with add-on boards, then onboard and finally on-chip (this is what Intel AMT does, when it isn't busy running rootkits).

The little two-PC switch box the article refers to (I think) was always a spillover from the server KVM technology, except with a very small market and more difficult to make. Servers don't need sound and are fine with just VGA video. Home computers want USB (which variety?) and audio switched, or high bandwidth signals that are very touchy about interference or suddenly being connected to a different device like HDMI, Displayport or Thunderbolt. And then people want the stuff for cheap.

> signal switches with arbitrary bandwidth remain available: analogue relays

Have you tried that? Cut a HDMI cable in half and splay it out, then try switching that to another display. See what happens. If it were that easy, it'd be around (or you'd just be building it yourself).