I just interviewed at Google on Friday, and actually brought this up with my lunchtime interviewer. According to him, Google recognized this problem about a year or two ago, and rejiggered their promotion system to place less emphasis on product launches and more on long-term usage and stability.

If that's true (and I only had his word on it), it may just be that they're currently "flushing the pipeline" of all the promotion-fueled products that got started before this change went in, or maybe the change hasn't permeated all the way through the company yet. So it's possible Google's frantic pace of launching doomed products will taper off in time.