To the extent that futures act as the "hedge of last resort" you are absolutely right. It is hard to believe how much stock market liquidity is conditional these days, hanging on those few lonely futures contracts... ES acts like the Sun for the entire planetary system of large-cap ETFs (arbing SPY vs. ES requires a dedicated line between Chicago and New York, so don't try that at home...) Blow up that center of gravity (limit down anyone?) and all that conditional HFT liquidity will disappear without mercy or delay - no 8 minute grace periods here for you to escape (or is gravity instantaneous as well?