It's worth reflecting on the context from which Moto X, Moto G and Moto E emerged. Google had bought Motorola Mobility.
Rick Osterloh [currently leading Google's hardware division] was in charge of the entire product line. The result was raising low-end handsets to the quality of mid-range. The philosophy considered all aspects of the user experience including software. Things were kept simple; only three models. Updates were released quickly and regularly. It was a response to the poor quality of handsets being produced by OEMs, and sold very well as a radically different proposition.
But maybe it was never meant to be a long-term strategy - only an attempt to shake up the industry and improve the pitiful low-end [of that time].