As you can see this is an area of considerable interest for a lot of people, and I'd like an inexpensive modern one myself. For me, the DevTerm is too small.
We can exclude all of the 1980s DOS-based models you linked. In the 80s getting a computer made was a feat unto itself, a tilting LCD monitor would have sent the price to the moon.
So that leaves the last 3 links you provide.
#1: The screen is slightly tilted. So it isn't "flush with the keyboard" as I described. Also, this seems to have a typewriter-quality keyboard, which is the selling point there. I'm sure the users are touch-typists who are writing their novel, not someone trying to use BASH and VIM, which I'll have you note, is the target audience of this RISC-V computer.
#2: ...This is a keyboard. Which you connect to an external monitor. Sure, it has an internal monitor, but you can clearly see from the demo that it's meant for preferences and settings. You don't use it standalone. On-the-go seems to be a feature, but it isn't the focus, so it isn't a good example.
#3: ...This is a toy. This is a nostalgia talisman for people who want to go to a techno rave, or people who like the aesthetic of the Tandy 100. It isn't meant for practical use.
Some of those old models were very successful and were very widely available, meaning that they still are today. When I wrote the Register article -- in which I said that I bought both an NC100 and a Z88 to research them, both for about £10 -- I didn't know about the (American) Alphasmart machines. Once I learned, I bought both, and while the 3000 was twice the price, that still means about £25. Easily found, widely available today.
My Dana Wireless cost more: circa £50 with shipping from the USA. Still not a lot.
This falsifies your statement that "nobody ever wanted" such things.
Your disingenuous statement that a tilted screen does not match your criteria is also false: the point here is that there's no hinge. It's one piece, robust and solid and cheap.
All 4 of the devices I have possess "typewriter-quality keyboards".
You are trying to wriggle out by splitting hairs. It's a very poor try at that.
These things are real, they were commercially successful, sold in large numbers, were very popular -- and are widely missed.
While there isn't a cheap decent-spec modern one, that's largely because commodity pricing has made laptops so very cheap now. I have done an ad-hoc feasibility study with a friend and we glumly concluded that we couldn't make one in small numbers for less than the price of a perfectly good ChromeBook.
But I submit that the DevTerm shows there is interest.
You are badly wrong here.
I see others are pointing to the Tandy Model 100.
I will add the Cambridge Z88: http://www.computinghistory.org.uk/det/4860/Cambridge-Z88/ https://oldcomputers.net/cambridge-z88.html
Still in use, OS still being updated, spares and addons available new. https://www.rwapsoftware.co.uk/z88.html
And the Amstrad NC100: https://www.ncus.org.uk/intro.htm
I have both. I have written about them: https://www.theregister.com/2011/11/10/portable_writing_tool...
I also have both an AlphaSmart 3000 and a Dana Wireless: https://en.wikipedia.org/wiki/AlphaSmart
There was the QuickPad: https://www.techtoolsforwriters.com/tag/quickpad/
... and the DOS-compatible QuickPad Pro: https://www.victor-notes.com/qppro/
And the Fusion Writer: https://www.at-udl.net/Writer-Fusion_2223
And the Tandy Portable Wordprocessor range: http://www.computinghistory.org.uk/det/2720/Tandy-Portable-W...
In terms of contemporary modern kit...
The Freewrite: https://getfreewrite.com/
There's this modern peripheral: https://www.kickstarter.com/projects/ficihp/ficihp-multifunc...
And this device: https://www.hackster.io/news/the-ready-computer-model-100-bl...
As you can see this is an area of considerable interest for a lot of people, and I'd like an inexpensive modern one myself. For me, the DevTerm is too small.