How is printing more steem to make steem's market cap bigger than the total of the SBDs act as a sustainable way of keeping the debt in check? Printing steem should only work as a short term increase in the market cap. After that newly minted steem hits the market as a sell pressure, the market cap will decrease again, to where it was before, or more. It seems like an underhanded way of tricking people into thinking that there's a bigger market cap than there is.
Also, market cap is a poor way of figuring out how much an asset is worth, especially with very volatile assets. It is not a sum of how much people paid to get their share in the market. It's simply the number of shares times the current price per share. And it is not the sum of how much everybody could get out of the market if they all sold - by the time most of them had sold, the price would be much lower.
And this is compounded by the fact that the blockchain is printing more steem everyday. As well as the fact that the steem created can't enter the market immediately, it happens over the course of two years. Market cap might be an "ok" way of figuring out how much debt a publicly traded company is good for (not really though, investors would look at revenue and capital instead) but it is utterly terrible for figuring out how much a blockchain, especially one with these kinds of rules, is good for.
SBD is not included in the market cap. I'm not sure where you're getting market cap from; I never talked about market cap in my video... The relationship between the SBD supply and the market cap is that if the SBD supply ever exceeds the market cap, the chain is running on a fractional reserve as the entire blockchain is not valuable enough to pay its debt. In practice, insolvency would come much sooner, as the mass execution of SBD contracts and sale of the resultant steem would drop the market cap considerably.
The chain has a lot of rules and formulas in place to regulate how much debt it's creating to try to maintain solvency, but at the end of the day, an SBD holder must decide whether he wants to hold that debt or would rather hold asset.