Does every single cavity left untreated end up needing a root canal eventually?or only sometimes? how does that work?

Yes: The speed at which it happens, and the degree of pain throughout the process varies greatly. Can be soon or take years.

Usually: In actuality, the tooth can decay so much that root canal therapy is no longer possible... there's not enough tooth structure to restore, resulting in the need for extraction.

Eventually yes.: An untreated cavity is the most common cause for this infection. Even one cavity left untreated can lead to a tooth abscess (infection) or destruction of the inside of the tooth, ending in a root canal or tooth loss by extraction. Remember your primary goal is to save your teeth.