It may not, though I'd be surprised if nobody hacks together a sliding keyboard for this thing. It'll probably be bulky. The fragility issue can be solved by putting a good case over the whole thing. Assuming the overall platform works out to a limited number of sizes, I'd likely offer to sell cases made of various fiber-reinforced polymers[0] with cutouts in user-specified locations.
The Sony Z1 Compact is approximately the size of a Moto X despite having a smaller screen. I think the waterproofing adds some bulk. The Moto G also has a 4.3" screen and is so close to the Moto X in dimensions that it fits comfortably in a case molded around a Moto X, though some cutouts don't align correctly. All of these are a bit too wide for me to use comfortably one-handed, while the iPhone is not. I don't expect modular phones to address that problem effectively, but I wanted to point it out as a segment the current market does not address at all. Based on totally non-scientific anecdotes I've collected by talking to people online, it's the largest of the three segments I mentioned.