Well, there is a fundamental difference between some of these retired API in the past and this one: in the first case the developer is the user adopting the AP

Well, there is a fundamental difference between some of these retired API in the past and this one: in the first case the developer is the user adopting the API, and enhancing his own service with it, without the end user noticing it. With this new API the end users of the product will be people with google accounts which will be able to seamlessly interact with 3rd party products. This greatly benefits the company as a platform, which seems to be the recent focus (which led to unpopular unfortunate choices as dropping "leaf" products). It's ok to criticize that choice etc, but that's another topic. I guess that if you want to build an application which leverages on the google's ecosystem, it's a reasonably safe bet it won't be dropped soon, as google has a immediate return from it (unlike translate API for instance).