I don't believe it's possible to put buy or sell orders at 0, so this is not something that will happen, but it can hit the lowest possible unit. But realistically, at that point people would have abandoned the token completely. You'd have plenty of accounts holding significant amounts of STEEM that are worth nothing because nobody values it. But think about what this means. It means the blockchain itself would not be operating at all due to inactivity. By this time, none of the websites interacting with the chain would exist. The only way to tell what used to be there is through snapshots through the wayback machine or if some fellow stores a copy of the now-defunct blockchain data.
That's what it effectively would mean for STEEM to "disappear", forget about price.
It's also interesting to speculate what events would have conspired to get to that point. We already know another platform that went through such a nose-dive and is near the point of being shuttered: it is called bitconnect. As long as there's some amount of believers, it can maintain a very low price and low activity. But at that point, people have moved onto other platforms that stand a better chance at persisting and being used in the world. And we have others that are completely dead. No nodes operating.
Not sure if this answers your question, but that's what it'll be like. If it gets to these situations, barring some miracle the platform will effectively be dead.
We can also talk about how the debt ratio would bend as it nose dives, but this probably isn't so important to the question other than it being fun to think about.