I don't see the point of this.
A smartphone is literally a handheld computer with more capability than this. Save for game controls, there is very little that a touchscreen doesn't completely replace with full flexibility (besides, unless it's the size of a Gameboy, the vertical controller layout sucks).
This isn't a new concept either, as I'm pretty sure I've seen two phones and one whole laptop using this exact drop-in modularity gimmick. They all failed.