Very good point. It would be nice if in the future Steem could add a feature to verify an account through e-mail, with verified accounts have more perks. Additional perks could also be added according to the account's seniority. But as of now, if you have set your private key and could sign-in (through Facebook maybe), then the account is definitely yours! But, further verification methods through e-mail or multi-sig/authy would be also appreciated!