interesting thought. though my knowledge of software-defined networking is limited, i see parallels, but also differences.
that bottleneck you mention, i sooner see them build better controllers with more power to overcome the issues. sure solutions like instant have there advantages, but will they become the dominant solution, doubt it. also typical to see how back in the days there was a similar trend with the fat APs without controler, which then got replaced with the control based wireless network.