90%-10 is a theoretical number, the ideal ratio of free to captive steem (at least as the white paper authors reckoned it). its actually, if im reading the numbers correctly, somewhere around 97-3.
total vesting steem
114,502,296.013 STEEM
Total steem supply
118,370,886.624
That said, a 90% drop in the $ value of steem could break the system, or at least be the first step to it happening. Becasue the same steem that fuels vest value is also used to fund the redemption on SBD.
So if steem value decreased by 90%, and there was a run to convert SBD that were bought high, that would get your SP balance running the wrong way... Especially if there was a teddyKGB type figure involved buying up the debt and then attempting to sell it all at once.
It wouldnt break it for sure... but it would make it vulnerable, at least for a time.