Dunno what made me think of this just now. When I worked for IT in a school district way back in the 90s, a librarian told me she kept a supply of mouse balls in her desk because kids would steal them out of the school computers. What I remember about those balls was they picked up dust and crud off surfaces. Pretty soon optical mice came along and they were history.
The keycaps thing is a huge problem now because of the widespread use of Chromebooks with butterfly switches. 9 times out of 10 if one of those caps is torn off, the switch is permanently damaged and the whole keyboard needs to be replaced.