Isn't that what archives are for, though? But then I was thinking about how, e.g., the Modern Tales webcomic site is growing in value each month as the archives get larger; why subscribe for a year rather than waiting eleven months and subscribing for a month and reading the archives? (Maybe you prefer reading it daily, but I don't; I'd rather read a completed story through from start to finish.)
And it dawns on me maybe this really is the way pay sites of the future will go. They will add new offerings (web comics, humor stories, porn) every day or week or month, in a very regluar fashion, until two years after you join it all runs out. It doesn't matter when you join; the list of available content is tailored to your start date so that you get exactly two years (or whatever) of stuff.
Subscription sites are already undercutting the linkability of the web--URLs aren't Uniform after all, in the sense of working for everyone (which is probably not the intended meaning of Uniform anyway); but this will just make it worse, as different people with subscriptions to the same site will have access to different content.