Surely whether a book is under copyright or not has some effect on the publisher's decision to publish, the price at which it is published, and the consumer's choice to buy. You seem to be saying that there's no way these effects can be significant, because the top books will always sell and all other old books will never sell.
I don't think there's a clear dividing line between the "top books" and the rest. The line is fuzzy, and will shift in response to market pressures. For a random example: The Production of Commodities by Means of Commodities was a rather influential book in economics. It is out of print and listed for over $100 used on Amazon. David Ricardo's Principles of Political Economy, which probably appeals to the same smallish group of econ geeks, is available in a reprint for $10.
I don't think there's a clear dividing line between the "top books" and the rest. The line is fuzzy, and will shift in response to market pressures. For a random example: The Production of Commodities by Means of Commodities was a rather influential book in economics. It is out of print and listed for over $100 used on Amazon. David Ricardo's Principles of Political Economy, which probably appeals to the same smallish group of econ geeks, is available in a reprint for $10.