I have an alternate take on this topic. Language is constantly evolving and there is no original, correct version; you could go back to any point in history and find people upset about the appearance of new usages which today we consider correct. How does one even determine what correct English is? The dictionary is itself empirically based and simply reflects how people are speaking and writing.
I also think that the sense that the language is going downhill in the recent past is an illusion. The writing that survives from earlier eras is that of the most well-educated, literate people of the time, whereas in the modern era we constantly read and hear the writing and speaking of people of all levels of literacy. On the whole, this is an improvement – our society is much more literate now, in terms of the number of people who can read, than it was in the past. This means that we hear more from people who are less well-educated, but in reality the average person is probably better educated now than back in the days when we imagine everyone to have been speaking perfect, proper English.
I do find it annoying when people are just sloppy or lazy, but I think one of the reasons that English is such a rich, complex language (it’s generally estimated to have half a million words, by far the most of any language) is because it is so adaptable. The English we now speak began as an Anglo-Saxon language and was later heavily infused with Romance elements, so that it was, from the beginning, an amalgamation.
Coming back, then, to “I could care less” – it is completely backward, of course, but personally I enjoy that about it. It’s funny; it’s weird; it’s the kind of strange quirk that gives writers in English so much to work with.