The problem with Bitcoin is the last one. To be comfortable to use as medium of exchange, it needs to become practical to use at a scale of hundreds of millions of people. But, even after 15 years of active research, it still cannot satisfy this criterion. There are other second layers developing as we speak, which will presumably solve the scalability problem to a great extent, but self-custody of Bitcoin will likely remain less appealing compared to the convenience of entrusting it to third parties, like banks.

This is the true cost of self-custody.