Earlier quoted context omitted.
Google could fix this credibility problem in a single step by simply saying "We won't materially change documented functionality of services A, B, or C in the next 5 years. Upon breach of this promise, we will pay any affected user $10M". If they did that, I'd trust them. If they breached my trust, I'd get a cool $10M. They wouldn't shutdown the service because of the extreme financial disincentive to do so.
That opens them up to a ton of liability (what if something unforeseen happens), for very little gain. I can't imagine any company doing something like that.
Providing credible hostages is a good way to build trust. Such a promise would be fairly credible if drafted properly.