For the financial arguments: Treat BTC as if it was in the "precious metal" asset class, but digital. Volatility is still an issue, but it's still a pretty new technology. As a technology, I agree that it's not that impressive. Stop making so many dang vaporware altcoins.
The problem with the argument is it always ends up circular to some extent. I always see something like the below - where crypto enthusiasts always try to argue bitcoin is something different than the current argument being levied against it... Crypto Enthusiast: "Bitcoin is a currency" Crypto Sceptic: "Well it's not a very good currency because of X, Y & Z reasons, and nobody is spending it" Crypto Enthusiast: "Well…
There has never existed a currency that has had such properties previously.