Well SD wouldn't be debt, because they are earning a (currently 10% APR) interest rate. I guess we can think of liquid STEEM (i.e. non-SP which is not SD) as debt because it has a much higher rate of constant debasement loss, so it is mathematically similar to a loan to buy some stable value commodity where the loan is being paid by selling off chunks of that inventory, i.e. your SP value being depleted by interest rate. But most loans aren't paid back that way, so I think the analogy is confusing.
I prefer to think of liquid STEEM as bad money and SP as good money, in Gresham's law. Remember bad money drives good money out-of-circulation. However that isn't entirely apt either, because SP is not entirely good as it lacks liquidity. So if one could circulate SP, it might be the case they would circulate some and not only liquid STEEM.
Rather I think the most apt analogy is liquid STEEM are Argentina/Venezuela pesos and and SP are time-locked CDs (certificates of deposit). Yet those still aren't entirely accurate because these are not just currencies, they are also equities in a highly volatile asset class (small cap altcoins).
Thank you for explaining that extensively I see your points. I have similar worries about this altcoin, which are only intuition based on aspects of beeing afraid it would be depleted from it's market value soon. But your presentation made me realize I have much to learn. Thank you again for sharing your knowledge and thoughts.