Additionally, the SMTs are still running off the Steem blockchain which is maintained by the witnesses who receive rewards for creating blocks with STEEM. If STEEM goes to 0 because everyone is now using SMTs then there's no more incentive to run witness nodes and the whole system stops.

In that case, the Companies implementing SMT will run nodes so that they have an influence in the system and keep the system running. But that would lead to a bit of centralization but the system will keep going on.