FYI Golos(Steem fork) has implemented the compulsary convertion, as it was proposed earlier by Dan Larimer here.
https://steemit.com/steem/@dantheman/steem-dollar-stability-enhancements
However my favorite way to withdraw SBD from circulation would be introducing the option to "power up" SBD.
Well the compulsory convertion can be a big problem if you factor in the amount on exchanges. The "power up" option has the disadvantage that it doesn't affect the debt ratio or it can even make it worse if it is incentivized with "paying interest".
Once "powered up" the corresponding amount of SBD effectively "does not exist" any more, from that moment on it is just an extra line of code to calculate vesting_shares.
Therefore it shouldn't be taken into account for calculating the debt ratio.
If the amount of the liquid SBD is below the haircut line you could power down SBD as well.
Otherways you power down SBD to Steem taking in account the haircut.