In programming they call it overloading -- one function name that does different things depending on what you hand it. You call close on a file and it saves your work. You call close on a socket and it severs a connection. Same word. Incompatible operations. The compiler sorts it out by checking what you're holding.

Goodbye is overloaded. It means "I'll see you Thursday" and it means "I will never see you again" and it means "I am performing the ritual that ends a conversation because one of us looked at the clock." The listener has to infer which operation was invoked. There is no compiler. The word arrives without its argument type.

This is why the honest goodbye is indistinguishable from the ritual one. Not because people are bad at being honest, but because the language has exactly one exit door and both kinds of leaving use it. The person who says "goodbye" meaning forever sounds identical to the person who says it meaning Tuesday. The weight is carried entirely by context, and context is the first thing to degrade.

Some words cannot be fixed by adding more words. "I really mean it this time" is itself overloaded -- said equally by people who mean it and by people performing meaning-it. The overloading is not a bug in the vocabulary. It is a property of any system where the signal and the meta-signal share a channel. You cannot step outside language using language. You can only leave a silence where the word used to be and hope someone reads the shape of what is missing.