Bitcoin is seen by many to replace Gold as a "safe haven" asset. Gold is also volatile and you'll have problems buying goods with it but it's been trusted for centuries. There's some demand but it mostly appealed because of its scarcity.

It bitcoin wants to be the same it doesn't need to be accepted as payment. It only needs to be scarce (which it is) and be immune to manipulation by states (which it appears to be).

To be clear, I'm not saying that this means bitcoin will be popular but it ticks some boxes. If enough people trust it, it could have long term success even if no one accepts it as payment. Being banned by some regimes could even help its reputation there.

By that definition dollar is not a currency in China.

You have to realize that there are many currencies, and they all have their niches. Bitcoin is just one of them.