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: