If you want to replace signers of a Bitcoin multisig you need to move the coins (potentially $1 million worth in this case) to a new address with a new multisig. To automate this requires some sort of hot wallet software, and which all of the signers would need to have online. This is not trivial and presents significant risk. It would need to be very carefully developed and audited.

Also trading between BTC and HIVE/HBD has to be done somehow, probably through an exchange (which means someone probably has to KYC and interface with the exchange) or perhaps DeFi (adding additional issues, probably including more signers/wallets, and the fact that all Hive DeFi solutions involve a centralized custodian afaik, so further trust required).

If this is a theoretical discussion all sorts of things "could" be done, but if the goal is to actually accomplish something, grasping at challenging decentralization problems when you can get by with something reasonable to accomplish the function is just a huge blocker.