IMO the overall pool has to in some way be fixed in STEEM as that is the only way to have it be a set portion of inflation (where ever that comes from). So the amount of SBD that could be created would be effectively capped by the amount of STEEM going to the overall system, as is the case with current reward payouts.
I guess that means that some items at the bottom of the queue may not get funded because the available (SBD) payouts are not sufficient if the STEEM price drops. In turn that could create some issues of people being willing to take on work when their funding may get cut off if the STEEM price drops. I don't think this is avoidable in a system with finite resources.