What is the use case of bitcoin?

There aren't any.

I think only contract coins like Ethereum will worth anything in the future, since there are existing use cases for those, like auditable real estate registrars or identification purposes for people.
I certainly don't see "value coins" like bitcoin growing in the future. There is nothing really to believe in them. The price is too volatile for them to become a gold-like long term investing asset.
In 10 years probably bitcoin will phase out. There are virtually no use cases for bitcoin for an ordinary person. It cannot be used for day-to-day transactions nor as a long-term investment.