hmm,
I like the basic idea. Of course, ending a campaign with: Sorry, did not matter at all would be a total desaster. But I think you could create a fine campaign and a superb ending, out of the basics.
A long campaign of searching hints and following those hints. Speaking to members of an old order who seem to know how to find more information about this weapon, sending the PCs into the darkest corners of the world... then you find out, there is no "ultimate weapon" or what ever. But the great evil is still there.
The PCs will realize, they, allongside of all other heroes of the world (who should be of a lower power level than the PCs by now) will have to start a fight, allthough they will not stand a chance. (Or so they think, they spend the last X sessions finding clue after clue how undefeatable the ancient evil is...) but being D&D Character of maybe Epic Levels, they actually will have a chance to win this Epic fight.
I can imagine my group RPing the walk into their doom, suspecting this battle to be the end of the campaign... and winning. Would be fun, I think.
Later on, they might even figure out, that what they have to do now, is create a new big riddle (Like creating a new order of schoolars who know just enough to send the new heroes to the dark corners of the world), an endless quest, able to train and equip heroes in the far distant future to be able to defeat the ancient evil again...