You also get the benefit of instant access to a huge catalog, though. Sometimes you just want to look up a quote in a book you read last year, or get a sample of an author that was recommended to you, or look up the context of a passage you saw quoted online, etc. That alone is not worth $10 a month, but it does add a little value beyond just reading full books.
If it also has non-fiction books - which usually are very amenable to skimming , or to reading just interesting parts , it's a very good value for $10.
I think the word "problem" is a bit of a stretch. It may not have as much actual value -- dollar cost of books read in a month/$9.99 -- as the perceived value -- unlimited. However, a successful product is about perceived value, not necessarily actual value. As the parent comment mentions, that convenience of looking something up without worrying about costs or alternate searching, and the freedom to not have to worry about the price of individual books, or having a shared kindle unlimited for a whole family with kids you want to encourage to read, are all worth something to a customer that can't be assessed in dollars. And of course, it's up to the customer to evaluate if they actually have the needs fulfilled by the product.
It's definitely a brilliant business move on Amazon's part.
