A very common objection in ethics papers is something like, “everyone else is / isn’t doing X”… which implies that the right thing to do is that thing everyone can / will go along with.
It’s an important question about “can” — if it’s impossible for someone to do what you think is the moral thing, then it seems pretty pointless to say they ought to do so — At this point, you probably aren’t surprised to hear that “ought implies can…” is controversial among philosophers…. because, the limits of “can / can’t” are pretty soft. So, to say someone “can’t” do something is often more like they don’t want to do so, they aren’t able to see how that thing is possible etc…
By way of contrast, I can’t fly without an airplane, I can’t breathe under water etc, I can’t write a check for a million dollars (well, I could, but it wouldn’t clear) etc… so, in that sense of “can/can’t” ought implies ‘can’ makes sense.
As for “everyone won’t do X” — that’s another story. Human history is full of folks who went against the grain, who saw what should be doing but wasn’t, who stood up and told it like it is etc… because, the thing is that what’s right often isn’t what is popular, what will become a law etc.
So, if you’re tempted to make an objection like this — think more about it… can the objection become something more like ‘it’s against human nature to require this?’ — if so, you need to explain what about human nature contradicts the theory.
This kind of objection comes up in terms of things like the ethics of warfare. As y’all may or may not know, that’s what I wrote my dissertation about — so, yea — I’ve thought about it. The thing is, the ethics of warfare are based on the ethics of the society doing the fighting — so, even if the other side acts in the wrong way, that action may or may not be in line with their own set of values — but, if it contradicts the values of the actors on the other side, then the other side ought not respond with similarly wrong acts.