My own opinion is that humanity should keep track of pretty much every useful piece of information -- books, movies, games, scientific data -- it can get its grubby little hands on. Because once it's gone, it's gone forever, no matter how badly future generations may wish it was still around.
Therefore, in the event that the new release is a functional equivalent of an older release (same exact graphics, music, story, etc.), just updated to run on modern computers/consoles, I would consider it the same game, and worthy of purchase if you want to access the original. But if pretty much anything has been "updated" or "improved", I consider it a separate version of the same game. It still may be worthy of purchase, but that doesn't mean that the original can be swept into the trash heap of history and be forgotten.