Bitcoin is just an example for an attempt to create hard money.
What happens to money in the long term doesn't matter much, what happens in the short term does. Investment vehicles short-term fluctuation matters less, but their long-term performance matters more.
> That leads to inefficiencies in the market, and in particular to less sustainable behaviour of people and less sustainable societies.
I see no reason to believe that the long-term smooth decline in the value of money does that at all compared to achievable alternative behaviors of money. In fact, one of the reasons money is typically managed with a goal of low-but-positive inflation with low volatility is that there is quite a lot of experience suggesting the opposite.