Their usefulness scales beautifully with experience and developed skill
Someone who knows only a little bit of numbers and letters can use a keyboard for a variety of tasks, even without knowing about the importance of the "shift key". As each button on the keyboard is labeled clearly with what each key will output when pressed, a person can guess quite quickly and accurately what each thing does. And when you don't know? Well, just don't push it. Or, go ahead and push it - see what happens! And Shift + Key = mind blown! (Wait until they learn about CTRL and ALT! Or, ok, CMD and OPT.)
Yet with time and experience a person can, with constant "payoffs" along the way, develop into a touch-typist that can type fast enough that the speed of their fingers is no longer a constraint on their communication. They will have to pause and think - and so the maximum speed of the device does not constrain the experience of the vast majority of human operators.
The Keyboard Isn't an Island - the Software Matters Too!
In a discussion like this it's easy to forget about the fact that a keyboard is NOT a typewriter - it's an input device to a programmable computer! And this allows the experience to be modified way faster and more cheaply than new devices can be manufactured.
It's easy to forget, but some of the killer features of computers that made them what they are to day was little stuff like word processors, spreadsheets, and databases - boring stuff, but stuff that's way worse to do by hand.
Yet when people came up with new ideas of what a computer to do, they could just make new software - no new hardware needed! A keyboard and a mouse and that's all they needed to create the internet browser and - when you add in a camera - add poorly spelled text to cat pictures. Spell checkers use to be important, anyway.
Letters, Numbers, and Directions - What Else Do Humans Care About?
Another slick power of the keyboard is that while a standard keyboard now has 108 keys, and custom keyboards can have more or less, what do most keyboards now have in common? Letters, numbers, and 2D direction keys (up, down, left, right). Add a few special function keys, and that's it. That is alot, but consider all they leave out!
Keyboards Don't Waste Effort On Irrelevant Input
In terms of human communication - which is what modern computer use is about, really - what else do you need to express? Does a human need to express anger with their voice volume to a computer? Will making a sad face help the computer understand what you are trying to do? Will hitting the keys harder somehow guide the computer to understand what you really wanted it to do?
Should a computer consider how you are dressed, or your posture, or your skin tone, or make some educated guesses on your socio-economic background and gender, to better figure out what you want from them? Would people even want that?
Keyboards don't allow you to input all kinds of things, and this isn't by accident. It just turns out that the things you can't do with a keyboard happens to generally be things you can't - or don't want to do - with a computer.
So keyboards are great at allowing us to do the things we want with a computer, and the things they can't do so far have pretty much been things computers can't do anyway.
Keyboards Have Changed, and They'll Keep Changing
When we think up new stuff, we create specialized devices - cameras, microphones, and biometric scanners, for instance. And if and when things get sorted out with the tech, they often get integrated back into the keyboard (like mouse pointer nubs, touchpads, fingerprint readers, etc).
Perhaps their greatest strength is ultimately their own limitations - you get 108 (+/-30) keys, press them or don't. And it turns out that that is enough for most people, most of the time - they usually need a bit less, and rarely need many more.
And so we have the humble keyboard, where the most common variety still ignores texture, pressure (beyond on and off), sound, vision, weight, and the inexorable march of time and raise in entropy that damns us all. And this obliviousness makes computers easier to program and use, and so the keyboard will be with us for a long, long time.