Thanks for asking a great question!

After thinking about it for a day or two I think it is exceedingly unlikely the government will overtly seize the bitcoin backing the ETFs. At least not right away. First they will deputize Blackrock and whoever else into issuing paper bitcoin instruments. Due to the absolute size of these wallets this will have a serious negative impact on the spot price of real bitcoin. At least for a while. Just like fiat currency manipulation, this will drive a black market price for bitcoin that exceeds the official sanctioned price. This scenario could continue for an extended period. The long game is the same, however. Eventually the USA will need a bitcoin treasury and these honeypots are simply too big to ignore. We all need to prepare accordingly, especially the ETF share owners.