This will fail not because it's not a good idea, but because the implementation is flawed.
Ryan Seacrest (yes the Ryan Seacrest) bankrolled a startup 10 years ago with an almost identical product. (They were sued out of existence by an already dying BlackBerry.)
I remember listening to an interview where he explained they restricted the product to smaller iPhone models because user testing showed the product didn't work well in larger models - the increased weight of the larger phones caused too much of a bending moment whilst holding the phone by the extended bottom, making it extremely uncomfortable to handle and not conducive to typing. It was therefore restricted to the iPhones 5 & 6 only.
Recall QWERTY phones of yore were literally half the size or even smaller than the models this is targeting. I recently found an old BlackBerry cleaning out a junk drawer and was shocked by how small it was. It would fit inside my current phone and remember these phones had removable, user-replaceable batteries.
Not to mention this looks much cheaper quality than Seacrest's forgotten startup produced. Perhaps it's the children's toy-inspired design asthetic.
This will fail.