If it's anything like Googles other APIs, people will build applications on top of it, and then Google will decide to shut down the API with no notice.
That's incorrect. This is a Google Cloud Platform service and when it reaches General Availability (GA) it will be subject to our Deprecation Policy. Just like Compute Engine, Cloud Storage, etc. requires us to give at least a 1 year heads up.
It's nice there's a policy around that, but I can understand the fears of someone considering using this to start a product - or even worse, a business.
Google has an history of shutting down useful products, why people should trust that one for long term integration?
Because we don't have a history of violating promises like this when done in writing? Seriously, I'd love to call us just "Cloud Platform" so you don't have to think "oh yeah, those guys cancelled reader on me" but if you look at the Cloud products we don't play games with this (partly because we hold ourselves to our binding Deprecation Policy, but mostly because we really care).
Google Search API, Autocomplete, Finance, Voice all closed with tons of active users. I'm not blaming Google; they were acting in their best interest, but the consequence is less enthusiasm for building software that depends on their APIs.
IMO a better option for Google, when considering to close an API, is to enforce payment and hike the price enough to justify maintaining it. If and only if enough users drop out with the higher price, then shut it down for good.
Even outside of services with a formal deprecation policy, Google rarely shuts anything down with no notice (their frequently cited shutdowns had long notice.)
Disclosure: I work on Compute Engine.