I don't think button is the correct terminology here because keycaps (or whatever) are a component of a button but do not serve the purpose of a button by themselves...
In English, "button" can refer to an entire switch mechanism, including the part you press, or to
only the part you press:
button [4.] any small knob or disk pressed to activate an electric circuit, release a spring, or otherwise operate or open a machine, small door, toy, etc.
And language is so tricky, you can even go the other way. A doorbell has a button, but people often say "press the doorbell", thus attributing the entire device's function to a single part that neither conducts power nor produces sound. :?)
Computer documentation has always referred to pressing "keys"; but when referring to
parts, the industry was always quite clear that a keyboard "button" was what you pressed to activate a switch. But because it's a specialized hardware term, rather than an instructional one, it's hard to find historical references to keyboard "buttons". Before the MK revival, few users had any interest in, or reason to refer to, keyboard
parts. To consumers, keyboards were always complete items—not the artistic, customizable, specialized things they are now.
Semantically, a "cap" is something that covers something. That's what defines it. With rare exceptions (e.g. the ones with built-in can holders and straws), a "cap" is something you wear to cover your head. Like keyboard buttons, wearable caps come in many different styles, and can be quite artistic. But as far as how they
work, they're head covers.
So if you refer to the parts over keyboard switches as "caps", you're describing them as
switch covers. But they're not just switch covers, are they?:
• They function by being physically moved.
• They have discrete operational characteristics (stiffness, travel length, smoothness, tactility, clickiness, sound).
• They have legends that indicate specific functions, transcending mere design.
• The material they're made of, and their shapes, affect how they feel when you operate them.
• They can be illuminated.
...and so on. They're much more than "caps"! (Hmm, I'm getting déjà vu here, so I'm sorry if I'm repeating myself... When you get to my age, you can never be sure.)
A "cap's" other defining characteristic is that it (typically) provides some kind of protection for what it covers. (Wearable caps protect people's eyes and the tops of their heads from the sun; the cap on a pickup truck protects what it's carrying; a pen's cap keeps it from drying out; a cap on a soda bottle holds in the fizz, etc.)
But to my knowledge, with the exception of IBM's well-enclosed buckling springs, so-called keyboard "caps" don't keep pollutants from entering their switches. So they're un-"caplike" in that way, too. (And yes, I realize IBM's key caps don't necessarily protect their stems—but they don't
have to protect the switches, do they? Besides, if IBM hadn't called them "caps", they would've had to call them something else.)
"Technical" is practically a synonym for "complex"—and the more complex something is, the more chances there are for ambiguities, misunderstandings and errors. So the guiding principle of tech terminology is: When a tech term's created, it should be done carefully, to avoid confusion with existing terms. And once that term's adopted and used, it should be used
consistently. Only when a thing innately changes (for example, when IBM created two-part buttons, and needed two new terms for those parts) is there a good reason to start calling it something else.
Anyway, I'm just speaking from my 40-plus-year experience as a tech writer here (and because I like to type, LOL). I see "cap" now, and I chuckle. It's just another case of the human "herding" instinct—how everyone can flip over to thinking or doing something not because it's right, but because a critical mass of agreement was reached. Our imitative, tribal nature has enabled us to survive for millennia, so it can't be all bad, even when it leads us to use incorrect names for stuff. :?) (Unfortunately, it can also do things like putting clueless, dangerous buffoons into positions of great public responsibility for which they have no experience, qualifications, aptitude—or even true interest, other than the opportunities it gives them to draw attention to themselves... But I digress.)
...I was actually just thinking the same thing. And it's not just about getting people at GH and other enthusiast sites to change: https://en.wikipedia.org/wiki/Keycap
What?! How could a
user-managed, user-edited online database possibly be wrong about anything?? Colour me shocked!! :?D