The main problem with smartphones is not that they started at an "in-between" time, but that they started in an "in-between" place as a hybrid of a PDA and a traditional cell phone. Cell phones tended to be very closed in large part due to the influence of traditional phone companies, which wanted control over every device plugged into the phone network, and smartphones inherited some of that.

Unfortunately, new devices tend to inherit from smartphones instead of desktop computers; even VR devices like the Steam Frame are clearly based on smartphone hardware, even though it doesn't have (for now) a cell phone modem.