As you rightly point out, ebooks cost less. They're also instantly available from (almost) any part of modern civilization. Hundreds of thousands of them can be carried on something the size of a thumbnail. Their text can be reformatted to suit individual preference. They can be easily backed up, and are not subject to physical damage or the ravages of time. They can be searched for specific passages. They're easy on the environment.

In short, the cost issue is just one advantage among many of ebooks over physical books, and I'm not sure why you feel that my argument was specious. Both products are available online, both have their advantages and disadvantages, and consumers are choosing ebooks. Why is that not evidence of the decline of physical books? It is not (in itself) evidence that ebooks are a superior product, as consumers may prefer physical books and just not want to pay more for them, but it's definitely evidence of medium decline.

Am I missing something? I'm not trying to be argumentative.