Here’s what happens to a really old wall switch:
A closeup of that broken contact:
This switch controlled an outlet, so I’m sure it’s hot-switched far too many vacuum cleaners, clothes irons, and suchlike over the last half century or so.
Our house is a bit fancier and originally had top-of-the-line mercury-wetted switches: the contacts sealed in the capsule don’t burn, but the springy supporting structures outside the capsule eventually wear out.
They’re still more reliable than X10 switches, though.