With current parameters and nothing else changing, assuming demand does not keep up with supply (I know this is not a widely accepted assumption) this is what happens:

1. SBD price keeps decreasing until we hit 1 USD
2. It goes under, but it now becomes desirable to use the conversion instead of a market swap, people will start burning SBD for STEEM.
3. If this isn't enough to keep propping the value up, witnesses implement interest rates to encourage holding.

The reason 1 USD is the threshold is because of the conversion rate of 1 SBD to 1 USD worth of steem based on price feed.

Note if the witnesses change the price feed to be against some other asset, the peg would then be towards that asset instead.