I find it kinda funny when people think they had Aldiun's coming planned out in what was it...1996 when Arena came out? The writers are just writers, not forture tellers. They had no idea they'd get this big and still be making games. I'm sure they were hopping for it, but they had no way of knowing. The fact that Aldiun's coming ties together the plots of the other games doesn't mean they had it planned out that far back. Its just the writers being creative with their job.
it has to be, the elder scroll that prophosizes Alduin's coming is the final elder scroll. If they make another game, it wouldn't be the elder scrolls any more.
The Elder Scrolls are Aedric Prophecies of unknown origin and quantity, the Elder Scrolls (whence the series takes its name) are, simultaneously, archives of both historic and future events.
Any person gifted with prescient powers is able to interpret the contents of the Elder Scrolls with practice. The information revealed about the future is never absolute. Once an event foretold within the scrolls is carried out in the world it becomes fixed within them.
Such insight into the inner fabric of reality comes at a price, however, as each new foretelling and interpretation strikes the reader with blindness for a greater period of time, while simultaneously granting them a broader view of the scroll's contents. Ultimately, the reader, having engaged in frequent acts of prophecy, is left bereft of their vision, forever after removed of their right to read the scrolls. By time-honored tradition only those of The Cult of the Ancestor Moth may read from the scrolls, the younger members caring for the elder as they gradually lose their sight for eternity.
The prophecies of the Elder Scrolls and the Heroes are interdependent, one cannot exist without the other.
Or in other words, read up on the lore before you say stuff like that. Its not the first time an Elder Scroll told of the end of the world. Its not the last one, and as long as Beth wants to they can keep making more.