Remember, the Winklevii are trying to set up an ETF so they can dump their Bitcoins. They need to find enough suckers.
I don't think hey are trying to dump their bitcoin. I think they are trying to profit off of bitcoin. They will obviously sell a portion of their bitcoin when the ETF starts trading, but I think they hope to be able to make most of their money from the fee that the ETF charges it's holders (the management fee). I would also not be surprised if a fee what charged anytime that an AP exchanges the ETF for BTC or BTC for the ETF.
ETF fee is normally 1-2%. Doubt they can charge more than this.
Kind of make me wonder, why they want to go thru so much trouble to make 1-2% a year. There are other way to make more money easily.