Provision of the service by Google:
Quote:
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4.3 As part of this continuing innovation, you acknowledge and agree that Google may stop (permanently or temporarily) providing the Services (or any features within the Services) to you or to users generally at Google’s sole discretion, without prior notice to you. You may stop using the Services at any time. You do not need to specifically inform Google when you stop using the Services.
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They even wrote it in their TOS. They can shut it down if they find that not enough people are using it.
That in turn will keep people from using it.