To what extent are "entitlements" really legally binding entitlements, in the US?
In Spain, to give you my direct experience, there is no such thing. Public retirement pensions do NOT constitute a binding entitlement that is immutable and known.
The government (Parliament) can (an constantly does) change them, and can at any time even pass a law cutting them by any arbitrary amount, or even completely eliminating them, regardless of how many years and how much you paid.
So there isn't really an "entitlement" to anything. The government "gives" and the government "takes" as it pleases (and is politically convenient).