Based on this, you could reach both of these conclusions:
1. Most literary fiction is inaccessible to the average adult.
2. It's a big problem that even moderately complex novels are inaccessible to the average adult.
The first statement (which I think is where you're coming from) is absolutely true. If you want to write a very popular book, it should be easily readable at a 6th grade level.
The second statement is more a statement of values. Some people (such as myself) find it problematic that the average adult can't read/understand a book that is more complex than Harry Potter.
You don't have to agree with the second statement. A lot of people don't. But I think understanding why someone might find that problematic is important. Personally, I think there are a lot of things worth knowing that can't be written at a 6th grade level.
If their goal is to write bestsellers, sure. That's where the €5 leisure novels come from. OTOH, if their goal is to push boundaries or be original, being subservient to the desires of the customers is counterproductive.
