There have been many theories about the end of the world. I have personally lived through three: Y2K, 2012 and the one that was supposed to happen for some reason on Sept. 23. I have known people who laughed it off and some who wholeheartedly believed the end was coming to pass. Which camp do you live in? Where there any that you believed in or knew someone who believed. Personally, I subscribe to the sun theory. The one that states when the sun dies and supernovas, we're done.
I might not believe in the kind of apocalypses you mentioned above, however, I find no reason to deny the fact that a "man made apocalypse" can take place. Especially in these times, it would always be good to just prepare yourself and have a plan of what you'll do if things turn south. However, always keep hope and try to prevent such events when possible. :)
I believe earth is stuck with us for quite a while! I do not believe anyone in the past, Mayans or otherwise, can predict an ending to anything. If our civilisation comes to an end it will be either a series of nasty asteroids, or because of our own mistakes: global warming, resource mismanagement, nuclear war, etc. But I don't think it will happen suddenly. I think if it did happen it would happen gradually which would give us a chance to try to stop it.
Earth will eventually deteriorate, be it Sun's expansion (which won't happen for few millions years, and who know what'll happen by then) or a man-made catastrophe. I think about this a lot, mainly because post-apocalyptic is among my favorite genres and there are so many works of fiction inspired by the potential of an almost complete annihilation of human life.