I agree with you logic about the Steem Dollars and that it gives you the best interest. The risk currently is the success of Steemit and Steem. If Steemit stands the test of time, this will be the best investment. If not, it will be gone. Currently, banks and fiat currency are seen as "safe" investments. I got high hopes for Steem and that it will work.
PS: Steemit is not the first cryptocurrency bank. That honour I will give the BTCPOP who got a savings account where you can earn a fix 5% income on your bitcoins.