Yeah, all of the keyboard phones on the market are ancient and/or total crap in every regard but their keyboard and/or super expensive. Some of them are all three at the same time (looking at you Planet Computers and F(x)tec!).
I believe I might have the most up-to-date and highest specced keyboard phone currently on the planet ;)
there is 100％ a niche market.
There totally is, as is proven by Unihertz still existing. But I fear, the keyboard market is a little fragmented, and just a handful of devices won't really capture it. People like side sliders, top sliders or portrait candy bar,. They might want a large or a small phone. They might like privacy/secure phones or hacker phones, or just cheap phones with a keyboard and nothing else. Or maybe a flagship phone with all bells and whistles and a keyboard...
The Fairberry is adaptable to pretty much any phone on the market (as long as it supports external keyboards). Foldables might be challenging though ;)
But that allows you to use any phone you like with a keyboard that anyone can make for ~€50, or maybe €100 if you get someone to do the soldering/printing for you.