How is this an unhealthy market?
Everyone can easily access the books they want. People choose to read the Bible and Harry Potter because that's what they want to read.
Of course, it might be a shame that we all coordinate on reading a few hits that everyone else is reading rather than check out some underappreciated books. But is the market really to blame for that? Amazon makes it easier than ever to find all the books. Publishers, as pointed out in the quote, are publishing tons of books that don't make it, in part because of a belief that it's a good thing to do. It's easier to self publish than ever before. And social media is often playing a big role in discovering the huge hits that everyone else is reading - many are not the books pushed by the marketing machine, but word-of-mouth successes that get discovered and shared by a huge group of dedicated, articulate readers.
If there were hardly any books selling 3-digit numbers, that to me would suggest an unhealthy market. The presence of lots and lots selling that many seems very positive.