But my point being, as long as your product is THE ONLY ONE IN THE MARKET, and as long as the market still exists, you have nothing to worry about.
If you think you are the only one in the market, there is a good chance that you have completely misdefined your market. History is littered with companies that thought they had their market down pat and that they were the only major dominate player in it. But there are always products that are just tangential to your market that make a good enough replacement that some consumers will start using (and then the company will improve making it good enough for other customers). See Blackberry, Blockbuster, large steel mills. Most companies will miss this until it's too late since the first customers to leave are usually the most price sensitive (and therefore usually the least profitable ones).
The only company that really catches onto this well is Apple who's fully willing to cannibalize their sales of one product to introduce a new one in a slightly different market. See the iPhone basically killing off the iPod and the iPad starting to eat deep into the Mac sales.