This is, of course, also an option, and I can also think of some cases historically where that happened. But really, there are usually at least a few who remain faithful despite the "Great Disappointment"* when a prophecy seems to fail.
*reference here is to the Millerites.
Perhaps not, but when do people ever do what is most sensible?
Iow, some will be dead, some won't. That would easily be true by the time of GMark (and, frankly, makes sense that the author decided to change what was likely the original claim from "none" to "some" for precisely that reason), but then, even if it weren't, are you arguing that whoever physically wrote the first draft of GMark had the luxury of not relating the original story (aka, "the truth")? That is was an option for him to change whatever he wanted to suit a subsequent non-event? Because, you know, new kettle o' fish.
Perhaps! I could think of a lot of reasons though, the most simple being that the verse was too widely known as an oral tradition to avoid its inclusion, so an apologetic explanation was already circulating to explain it away.