There's a slight issue with this...

Clef isn't free. It's a centralised login system that costs money. I don't know if it'd even be possible for Steemit to add Clef, being that your mnemonic password is required to generate your keys, not just a message from Clef saying "this person can login".

While it's a great idea, I don't think it could be done without handing over your private keys to a third party, which introduces whole new security risks of it's own.