The statement "Half of all Carbon 14 decays in 5,730 years" does not say anything about the decay in years after that.
Half lives on the other hand are defined as being a re-occurring phenomenon, such that it _does_ say something about future years.
Language does not work like code, yes, but that doesn't mean it can't have a clear and defined meaning. If it couldn't, language would be useless as a communication method.
Exactly, your first sentence isn't saying anything at all about the next 5730. Without outside knowledge of exponential decay, there's zero reason to imply anything about the next 5730.
If you instead say "the half-life of Carbon 14 is 5730 years" then you would be saying something about the next 5730 as well.
The English of "8 out of 10 businesses fail in their first year." is completely unambiguous.
Language doesn't work like code. Many interpretations can be sustained by a given text.