I don't think anyone but the manufacturer has an idea about a product's life. The manufacturer might not even have a clue either. Who would have predicted the 555 would last forever?
As an alternative, use a distributor that will send a notice for "last-time buy." DigiKey and probably many others do that. Unless the product is DC'd almost immediately, your company should have a good idea of potential sales and the amount of inventory needed to support the product for whatever time will be needed after that last-time buy.