Fungibility is a term used in economics to describe "items that can be mixed without differentiating the source". For example, if you and I pour a bottle of water into a pan, the water is mixed. We can't separate your water from mine.

Most of us view attention differently. A partner's attention is worth MORE than a random strangers. But narcissists see things differently. They don't care where the attention comes from, provided they get it, and enough of it.

That is why it's so easy for them to discard you when you no longer supply enough attention.