It doesn't sound like you know much about the market for books.
Sounds like they may know more than you in this case.
Note also that the average of 12 books a year is dominated by people who read zero a year, and those people aren't relevant to the market.
There’s a realistic cap on total number of books consumed by a large enough group of people to matter economically.
There is an infinitely high cap on the consumption of every good. That can't distinguish anything from anything else.
So...
> Would a figure of 60 books per year change the argument? 100? 200?
Yes, that's the difference between the argument being theoretically able to work on its own terms, or not.
Huh. No, no there is not. This is why a few rich people don't consume 100x as much toilet paper as you do. The cap isn't how much of the good we can make, the cap is human time and attention, of which time is fixed and attention is a highly competitive market. Simply put people are not buying infinite books and immediately throwing them in infinite fires.
1. All goods have capped consumption. There are no goods without this feature.
2. The cap might be very high.
3. If you want to argue that one good differs from another good by virtue of capped consumption...
3(a). It is not enough to show that a cap exists;
3(b). Instead, you must show that the cap is actually achieved.
4. The reason for this is that the cap exists for all goods.
5. Books do not come anywhere close to reaching their consumption cap. The consumption of books is just as unbounded as the consumption of immortality pills.
At the end of the day there is a limitation to how many books a human can read in a year/month/day/nano second. Add to that the number of people that consume massive numbers of books is positively tiny. Now add in that the number of books being written is increasing far faster than the amount of reading that is happening on average (which i believe is decreasing due to other forms of entertainment).
It seems that any product with effectively capped consumption will become a winner-take-all market. People go to the movie theater 6 times a year, so that's winner-take-all. People read 12 books a year, so that becomes winner-take-all. People go to approximately 1 university, so while an average university has 200 million in endowments, Harvard has 50 billion (250x). Heck, I'm pretty sure the same factor is leading to the rise of megachurches because people are only capable of attending approximately 1 service per week.
I recall the book Blockbusters by Anita Elberse (2013) being one of the first to point this out.