I agree that the externalities are still overwhelmingly negative, but one part you're missing is that cryptocurrencies act as a global reserve asset. Crypto is still tiny compared to other financial markets, but the existence of cryptocurrencies has probably reduced the volatility of other assets somewhat.
In the future, this effect will likely be more pronounced. It's an asset class that has some degree of independence from other asset classes, so it decreases the overall risk of the financial system.
The only good quality of Bitcoin is that it is completely decoupled from the economy. A bursting Bitcoin bubble doesn't take the rest of the economy with it. However, that only holds if countries don't use it as legal tender.
This isn't true. There are Bitcoin collateralized loans already, and there are Bitcoin backed ETFs. Companies have issued bonds underwritten with valuations that include BTC in the company treasure.
Bitcoin is small compared to the rest of the economy, and it's less integrated than many other asset classes, but it is very much coupled to other financial markets.
In the future, this effect will likely be more pronounced. It's an asset class that has some degree of independence from other asset classes, so it decreases the overall risk of the financial system.