Similarly, if someone sells you a physical product that is faulty, the seller is not required to produce a replacement, they can just refund you instead.
As well they cannot demand any such thing - the country's money is legislated to be accepted as a settlement for any kind of debt. It's the "legal tender" thing.
Hitting the Molly Switch on a DRM service is the exact opposite.
Digital goods should last infinitely, so a digital good that 'expires' should be fixed or the cost fully refunded (fully as there are still an infinite number of years of use left available; and that should be accounting for inflation too).
Seems fair enough to me.
Can I use the money to buy the same book? Is my time lost searching for this replacement worth nothing? If I added annotations, do they end up in the new book? If you answer any of these questions with 'no', its not fair enough.
You could have originally bought it on sell and that sell is no longer available.
You could have added value to the ebook by way of adding annotations.
The ebook could have had better display than its competitors, if it and all other equal quality displays vanished, they are now very rare. Someone would be willing to pay more than your original purchase price if they could now get that display experience.