My thinking is that people in his category (those who never learn to type on a physical keyboard) will increase. Unless you are a professional typist, the speed you can achieve with on-screen keyboards is good enough.
I'm sure more and more people don't use regular keyboards, but I don't see it as replacing a keyboard for lots of typing. I keep picturing someone needing to write a three page letter, followed by another large document, etc. On screen is fine for small things for sure and if you never have to do anything long, you could do that kind of stuff scattered in bits and pieces throughout the day. I often do it when out because I may be responding to something online and don't bother getting the TB out (or don't even have it with me in those situations usually). But if I'm having to do a bunch of posts, I want a keyboard.
I have sometimes thought about forcing myself to use voice recognition - because usually I can at home - to see if I can adjust my thinking and speaking process, but it is hard to make myself. When I'm typing, I can be typing one thing automatically and be thinking about the rest of the sentence. But when speaking, that process keeps getting screwed up.
But the keycaps don't magically change instantly, does it?
Few do, but, actually, at least on iOS for not, you can still use the keyboard extension ability to show the TB keyboard layout and it till show whatever it is you have the layout set to. You can even tap the letters on the screen, but that isn't too practical because of the small size and I'm not sure the combo keys work that way.
But I need some clarification on you typing with Japanese stuff. I've been questioning Treg testers who do Japanese and I have not yet gotten it all sorted in my mind. You say you need the keyboard characters to change, so you like the onscreen layouts so you can see those changes. But I assume you aren't having Japanese characters on those onscreen keys since there are, that, thousands of them?
From the others, I've seen them refer to typing the words in phonetically and then the computer system gives you various choices to match what you typed - so that gives you the Japanese characters, but you type in Qwerty (or whatever other system - I use dvorak). In such a case, I don't see why you'd need to see the characters change since you'd be typing normal letters. They also mentioned another approach, using kana and said there were only around 50 of those. Not clear on that yet, but I gather instead of typing regular letters, your keyboard would change, though I guess it would take alternative choices (like activating numbers and symbols) to get them all. Or scroll around.
From what they said, most typing on a computer use phonetic approach because they are using a regular keyboard, but on a phone, they use kana because it makes a better target for thumbs.
Anyway, any clarification you can give me would help understand the situation.