rPI has spawned a whole ecosystem of hacker-friendly hardware. Like Arduino, it generated demand for a particular form-factor - a lot of people bought rPIs simply because they were cool and cheap, then figured out how they could be useful.
The rPI has clearly moved towards being more powerful and power-hungry, with the latest model drawing about twice the power under load than the original. Documentation and kernel support remain a sticking point, especially for the off-brand PIalikes with Chinese chipsets.
The development of these boards is really dictated by the rest of the market. Nobody is spinning silicon specifically for dev boards, so they'll continue to use whatever cheap chips are on the market. A lot will depend on how well the foundries cope with <14nm process nodes.