You're absolutely right, Keychain is primarily a wallet and a tool to ease the use of keys on the Hive blockchain. Although onboarding is important, I do not believe it is our role to do that. Rather than onboarding, we tackle the retention problem, by giving users a safer and better experience using their keys.
To make a parallel with Ethereum,Metamask is not an onboarding tool either, it doesn't mean it has not an important place in the ecosystem.