To make the keyboard as bad to use with clumsy fingers as the TOHkb for the Jolla 1 and the N900's keyboard?
No thanks, IMHO a 5,2" screen and consequently a keyboard with its width is the smallest practically usable size (at least for people over 40)!

Oh no, that would be the perfect route for F(X)tec to kill their enterprise: Developing an Operating System.
Purism tries that, it takes years (by which such a device is delayed), bears huge technical and business risks, costs a lot of money and thus significantly increases the price of the device (by ca. €500 for the Librem).
Side note: I have some doubts, that Purism will survive this well, and am absolutely sure, the their software stack will not be practically usable for a power-user before 2021.

F(x)tec provides an open Bootloader plus no loss of warranty whatever has been flashed (as long as you manage to reflash the original Android software stack before sending it in), and I definitely do not want them to do more than that.
They are an excellent hardware provider and as such they already have to care much too much about low-level software (kernel adaptions etc.).

"Hardware kill-switches" are primarily good for those, who do not trust their software stack to control these peripherals properly.
If so, you should not use that software stack at all, thus implementing kill switches just increases parts count and mechanical complexity for no real gain.