Alot of the demand for paper money comes from the fact that you need it to pay taxes to the government, you can't use anything else.

what exclusive use cases does bitcoin have? internet black markets.

you can't use fiat on internet black markets because of the paper trail it will leave, you HAVE to use bitcoin (or some other crypto but for simplicity sake lets say its bitcoin because its the most liquid and has the most secure network in terms of hashing power).
bitcoin is the only way to send money over the internet anonymously, all other methods require interfacing with the banking system which is highly traceable.

so assuming the world drug business remains illegal, and most of it moves from dangerous physical black markets to much safer internet black markets like silkroad, that is very bullish for bitcoin.

we're talking about a huge industry that will have to use bitcoin and will create constant demand for it.