Current chase for thinness by manufacturers, even if it means sacrificing structural integrity[1] is laughably absurd.
May be I'm bit biased since I was frankensteining a similar concept with my limited capacity, but if the thickness of the device in this prototype is ~2.5" to 3" then the size of the final production device wouldn't be a deal breaker for many.
I would not replace my proper well working phone with a makeshift device like this at all.
And there are plenty of options out there already; No need to wait for this setup.
Like?
The only current mainstream Linux capable smartphone with physical keyboard I know of is Planet Computer devices[1]; even there, the first party support is for android and so the hardware is closed like any other android device out there.
Without this physical keyboard, you have phones like https://puri.sm/products/librem-5/#overview
Also there are plenty of non google service based android phones and alternatives.
You can also use Linux on most Android Phones as well.
A Pi Zero or a Pi in general is very very far away from what a Android based Phone does and can do. Alone for Security.
What is your thought why you would assume a lot of people could get beind this setup? I do like the project, its nice though but for me probably for different reasons then for you.