I disagree with you. Google is a private service which has no official contract with its ordinary users. Google doesn't simply "play for fun" with its algorithm but rather tries to gradually improve it - for the benefit of the majority of its users (who are people searching for information), while some (mainly companies and business owners) might be damaged by it.

If Google notifies specific users before it is doing major changes - as you suggest - this very act would cast suspicion upon Google, as an interested company which develops its algorithm according to commercial interests of its beneficiaries rather than according to the interests of the ordinary information seekers.