I hope im understanding this correct.
In the event that some global economic meltdown happens, and a lot of the investors (let say 30 %) deside to get real Bitcoins for the ETF´s they are holding, then we would have an equivalence of a bank run?
They're 100% backed.
In addition, one of the lubricants that keeps ETFs in general running smoothly is the 'Authorized Participant'; earlier in this thread, there's a discussion about how shares are created or redeemed via the APs, who either hand over large blocks of the underlying entity in exchange for shares, or who hand over large blocks of shares in exchange for the underlying entity. Ordinary investors trade shares, but they don't engage in the redemption or creation process.
What this means, is that the investors are not going to be getting any real Bitcoins for the ETF shares they are holding, meltdown or not. It doesn't matter how many try.
If they were Authorized Participants, they could get a fractional percentage of the bitcoin assets in the trust by paying a fee, and redeeming a basket of 50K shares. They would get 10K bitcoins minus the amount of bitcoins the trust spends to operate apportioned across the baskets, or thereabouts. Investors can only sell the ETF for whatever folks thing its worth.
The catch is, the trust can swap out for those bitcoins redeemed without notice for fiat, without notice at any time. So even the AP might get no bitcoin out.
This isn't unusual, most all ETF are like this. For most, no one would care that they get paid cash instead. For alternative currency ETF it can defeat the purpose of the investment (currency/gold/silver/bitcoin)