There are two cases. Keep the thing the way it was designed or change it as it not hard and fast rule that whatever you have designed in the past is correct or not changeable.
If SBD was designed to be always worth of $1 and steem to fluctuate then they should be kept that way. Reason is you have one currency which can be used by others/investors to manipulate while SBD will be kept the standard unit for payment dealing. This has long term effects.In the long run we will benefit from it when all the stores offline and online has started using SBD for the payment.

When it comes about the author and post rewards scanerio, think it as when SBD was $1 worth. Then we all survived and we were happy with that. Even that was the difficult time bcause you had very small community and not a very big market cap. So you might be earning less money now if this happens but it will give rise to the useage of steem blockchain resulting in the marketing of steemit and attracting new users. So then you would be able to earn more by having more followers and taking your content to bigger audience.