Unfortunately, I have a big mouth and would have said something if it continued. It may not necessarily be the right thing to do, but I've done it before. When I did, the offender shut up and didn't say anything more about it, but I know that it could get ugly if that person was a bit off the wall. So I admit that it could lead to a confrontation. These people think that everyone else agrees with them, which is why they go on and on. It's nice to be able to let them no that is not the case. But I really don't blame anyone for not saying anything. It may not be worth it in the long run.
Which begs the question, if the offender is not talking to you, is it appropriate to interrupt someone else's conversation if you are offended by the topic? It's really a tough call, especially on an airplane where everyone is on top of one another. If they were talking directly to me, of course, I would have no problem speaking my mind.