When SBD drops below one USD, bias and APR manipulation works along with choosing 100% SP reward. However, keeping the upper cap is the hurdle since there is no such mechanism right now. That's why reverse-peg comes in. Moreover, increased supply of SBD has low effect on price, since high speculation can overcome it. But STEEM blockchain has a mechanism when total SBD supply crosses 10% of total STEEM supply's, SBD production will stop. At some stage, with very high supply of SBD (20% debt), SBD can only be converted to 0.5 USD equivalent of STEEM.

https://steemit.com/steem/@timcliff/re-bacchist-re-timcliff-re-bacchist-re-timcliff-i-submitted-my-first-hardfork-pull-request-to-the-steem-blockchain-updates-to-the-sbd-print-rate-20180528t211227510z

However, it would be interesting to apply a negative APR when SBD trades higher than one USD.