I have a good sense of when I'm ignorant, and google very fast. If I feel the right mix of ignorance and curiosity, I will procure and devour relevant books. The trick is knowing a little bit about a lot of thing.

The honest answer is that I stop caring if I lose curiosity. The rationalized answer is that expertise requires maintenance, and a low likelihood of using some expertise doesn't justify that maintenance. For example, I am unlikely to ever do advanced financial planning, and I can re-learn / lookup the formulas for doing calculations, or maybe even re-derive them if I really need to. On demand learning!