Even if they act in good faith picking the correct chain, I'm missing details on what happens to the value in a possible Fork chain. When the BCH Fork happened, the value of the Forkcoin varied, but could have been sold for 10-20% of the Bitcoin value. If they keep that instead of sharing it with the ETF investors, it just adds to the list of reasons to keep your own keys.
I raised that same concern before, but only a few wanted to share their thoughts and opinions.
That part in BlackRock's proposal is dangerous because they, and the rest of the "ETF Cartel" composed of their fellow asset managers, could come together with a combined holding of more than 50% of the total supply in the future. I believe with such amount of the total supply, they could push their weight on the network and they might start making demands on what path of technical development Bitcoin should follow. Possibly start a hash war.
That's probably a Doomsday way of looking at the situation, but perhaps that's the right way of looking at the situation because there are bad actors everywhere.