This is the main issue that needs addressing.
The community/traders would have incentive to lower the price of Steem in order to get more Steem per SD. Simultaneously, this would incentivize the holding/buying of SD to sell at latest point possible.
The only clear solution and fair method to address this connundrum that I can see is for Steemit to purchase ALL SD at either market rate or preferably an agreed price which would also have to have the consensus of the community/stake-holders. We can't just take or force holders of SD to sell, however holders after a certain date would be holding them on a defunct chain.
The further benefit to this is that it would distribute Steem further, help further de-centralise consensus/influence and substantially reduce Steemit's holdings. This would be consistent with Steemit's stated goals and would address the further problem of perception and FUD currently circulating by detractors.
If this can all be managed, then I believe Steem has a great chance to become THE digital cash of the future. This would help marketing and general perception and adoption by the larger public I believe. Anyway, just my thoughts....