The premise is alluring, that's for sure, but I'm still confused about the implementation of the technology. The technical aspect are bit over my head, but I'm more curious about functionality and how it would differ from any other custodial wallet. Would "statechain" be a proprietary technology only available if using Mercury wallet, or would its licensing allow any wallet developer to implement it?
If it's the latter, I can see how it would be beneficial to bitcoin in general, but if its the former, I don't see how that would be any more attractive than other trusted custodial wallet providers who also offer internal transfers:
The internal transfer function lets you send funds between two Binance accounts. It will be immediately credited, and you don’t have to pay any transaction fees.
Mercury is a centralised service - but it's all open source (wallet and server).
Using a trusted custodian has very poor privacy, and your funds can be seized or frozen at any point. Also, you are likely to be required to submit to KYC.