I'm curious to know how you think a sudden hardfork by the witnesses would have played out in regards to exchanges.

Probably about as you predicted elsewhere. Steemit makes a new code release to restart the exchange nodes on the old chain, resulting in two chains (and points steemit.com at the old chain). Whether exchanges end up supporting both chains is hard to predict but has plenty of precedent in general. From an exchange perspective, chain broke (stopped reporting new blocks), devs fixed it. They're pretty wedded to the model of getting code releases from the "official" repo and probably would never agree to run code from some allegedly-community repo (mostly understandable) even if the Steem (sort of) consensus rules say that witnesses get to decide on hard forks. Yeah, that all seems pretty stupidly centralized but that's the crypto exchange view of the world 2019.

delisting Bitcoin Gold over the recent 51% attack

Delisting after 51% attacks is reasonably common, though not universal. Yes that is a different situation.

I suppose framing it as "exchanges are out to make money" and really don't care otherwise sort of defuses the entire idea

Likely a good first order model.

powering down to them

They didn't actually do that, which may say something.