I have seen that too about it declining sharply since the '70s because of mounting criticism. But I don't think that this is because other views have come to light since then. These other views have been around for as long as the appearing of Christ revelation (aka the rapture) has been around. (1st century). But, because people for the most part, do not want to be associated with an unpopular thing (yes, it still is Jr. High School for many of us), it declined on this basis as well. Cooper and Jones have done lots to foster this anti rapture sentiment. So much so that many fear to make a peep that they believe in such a "ridiculous" idea which, also according to these two guys (oh yeah, I forgot about Quayle) according to these three guys, by one saying they believe in the rapture, that would also "reveal" that they are among the lazy, sit on your butt, don't care about what is going on in the world, good for nothing losers these slanderers say we are.
I know it is common knowledge that Darby was the originator of the rapture. I've read numerous "articles" about that but this is not true. Although one I read last night about Darby I found interesting. They said that Darby was shown the difference between the prophecies given to the believers under the old Covenant and the believers under the new Covenant. I have no problem with that since I too have seen that in my own personal biblical research.
Just for the record, I did not come upon the belief of what is commonly referred to as the rapture from watching a movie or from reading other people's books. As far as I am concerned, from what I have read in the New Testament alone, I see it mentioned there very clearly (although many will disagree).
But, like I said, I don't debate these matters because it leads to unhealthy stuff. If the bottom line is love, which I do believe, then debating can prove counter productive to that and it almost always solves nothing.