Apple never promised a replacement for Shake. It was just the usual Apple rumor mill at work.
They bought Nothing Real for people and some technology. A few pieces from Shake were integrated into FCP and Motion, for example optical flow retiming (=smooth slowdown) and image stabilization ("SmoothCam").
Why would they buy out Nothing Real, sell the product pretty much unchanged for a few years, and then drop it altogether, when it was, at the time, a leading product in its niche?