simplicity and security, permissioned session keys (directly from wallet) rather than longed live keys in browser (for first iteration of 3id), in future goal is best of both, multiple accounts in 3id with the session key model
did interface is used everywhere, and can be swapped, but nodes verifiying need to support and most arent/wont for, unlikely to add many dids methods by default, esp if they require many diff crypto libraries
unclear yet, will be 3idv2 spec in future, but likely will be able to add existing did:pkh to a 3id
correct switching wallets/chains will have no data shared between them, they are written/resolved by did:pkh