In the year 2525, if man is still alive, he may find he has a web page from an unknown time period. To read it, all he'll need to do is reimplement all standards for the previous 500 years. Version numbers wouldn't make this easier at all, somehow.
And after all their efforts to get the unknown markup into an document form, they'll convert it's contents into the dominant information format of the 26th century: PDF.