They went with 28nm because it was the last planar (aka cheap) node around. GlobalFoundries recently launched their 22nm planar (22FDX), so it seems like there'

They went with 28nm because it was the last planar (aka cheap) node around. GlobalFoundries recently launched their 22nm planar (22FDX), so it seems like there's now a cheap path forward for making a new chip with an updated CPU design (and hopefully an updated GPU too).