You cannot prove a loss.
You cannot claim insurance.
With stocks you can do one, with art or buildings, you can do the other. This will make many rich people vary of holding wealth in bitcoin.
First, rich people are willing to take some risk. That's not going to be 100%, but it would make sense for someone to put 1-5% in an asset that has a lot of upside potential, despite the potential to get hacked. One million is pocket change for someone with a billion dollars, and is makes a huge impact on the exchange rate.
Second, eventually companies should rise that provide insurance against loss, probably to cover BTC banks with multi-layered, multi-redundency security. But I don't think we've seen anyone who has the reputation outside of Bitcoin to be trusted with that, atm.