You have to drop the idea that scarce/finite assets can, should or need to be used as "currency".

Currencies are necessarily able to be increased in supply easily.

Even though there are debates about the actual description of central banks expanding their balance sheets as literal "money printing", this is mostly a debate about where this liquidity flows. For the wealthy and corporations who can borrow at extremely low rates, of course this means they have access to this "printing" and will then inject this liquidity into assets.

>The gains can be explained by nothing else than mania and speculation.

Sure, sort of.

But finite assets are doing their job if they rise in price (in reference to fiat currency units of account) as fiat quantities increase, and the people who have access to this fiat look for a place to put it.

This is literally the use case of Bitcoin, and so it is absolutely "being used".

But since the expansion of BTC has no flexibility (strict supply schedule regardless of economic conditions or price), unlike gold mining or central bank open market operations, it makes BTC very volatile both on the way up and on the way down.

But long term (so far) it is doing its job (of absorbing expanding fiat liquidity and supply).