Value of USD decreases and increases relative to other currencies...in other words, the value of USD doesn't decrease in a vacuum, and changes in the dollar won't affect the value of SBD. It's hard to understand but the Balance has a good explanation https://www.thebalance.com/value-of-us-dollar-3306268 .
I personally don't see a problem with letting the value float. As in, not pegging to the dollar.
I'm still not exactly sure I understand the economics of steemit, but if I'm right that SBDs are backed in STEEM, there's zero structural problem with letting SBDs float. Whenever SBDs are more valuable than dollars, and people know about this, they'll use the platform more. Whenever SBDs are less valuable than dollars, and people know this, they'll use the platform less.
Meanwhile, we can take advantage of high SBD prices to acquire steem or steem power, and take advantage of low SBD to convert our STEEM or steem power to SBDs. The supply of STEEM is fixed -- it's not like anybody's going print more STEEM -- and as long as that's the case, the markets will, over time, probably swing back and forth.
(Assuming I understand the economics of steemit...)