"Theoretically STEEM could die off, but a SMT built on top of STEEM could survive... I think ...?"

I don't think so. Skimming the whitepaper it seems to me that STEEM will be to SMTs as Ether is to ERC20 tokens, so if STEEM is flawed and the blockchain dies, the tokens will go with it. Although, I suppose this doesn't say anything about the price of tokens. 1 ERC-20 token could be worth more than 1 ether.