Docs was google's only docs service, and subsequent acquisitions have been squashed into it (e.g. etherpad). Maps is google's only maps service. Blogger was more-or-less google's only blog service until recently, and I wouldn't count on it surviving much longer in the age of google+. Youtube happened the other way around, in that google killed google video and merged it into youtube.

Is play music more established than songza? I think yes, and google is unlikely to maintain both of them in parallel.

Waze is an important data source for traffic in Google Maps. They didn't want to lose that data by killing the Waze app.