In Steem people don't really have a choice - rather, the decision is made by the witnesses.

People always have a choice what code to run. If you don't like how things are working with the witnesses you can change the entire block production model if you want, and/or if existing witnesses don't participate in such a chain, the chain can limp along via backup witnesses until witnesses can be replaced.

Theoretically perhaps, but unlikely in practice. In any case, a chain that is not recognized by the ticker symbol STEEM would have almost no value

What chains will continue and what price they will have is always speculation. Perhaps informed speculation, but still speculation. I don't agree with you that anything not with the magic S-T-E-E-M imprint on it would somehow become worthless, were it otherwise of value.

Moreover it isn't even a predetermined matter what symbol applies to which fork. If the Ethereum community (miners, users, exchanges, etc.) had ended up not supporting the anti-DAO hacker fork as they did, what is now called ETC would have ended up now being called ETH, and Vitalik said he was open to that possibility at the time.