It's a mistake to say that even a Dead Metaphor has a single meaning. The meaning is constructed from the listener's experience (including with the conventional usage).
For context (heh), see Poem as Program.
For example, consider the Love Is a Journey metaphor. Part of Lakoff's description of the Love Is a Journey reads:
> “The lovers are travelers on a journey together, with their common **life goals** seen as destinations to be reached. The relationship is their vehicle, and it allows them to pursue those **common goals** together. The relationship is seen as fulfilling its purpose as long as it allows them to make progress toward their common **goals**. [...] (emphasis mine)
I've been married to Dawn for 35+ years. I've never conceived of our marriage as having a goal other than, I suppose, keeping the journey going until we die.
The interesting thing is that people who take different meanings from the same metaphor can nevertheless work together.