Main idea skill practice - I dnt understand

Anyone know why the last sentence is the main idea? In the simplification I jot down, I did write “writer = teacher” for the last sentence. But it wasn’t the first time this idea was introduced in the passage. It does say that there’s some sort of debate why there is increasing demand in college vs book shops. So, can anyone help me out?

The last sentence is where the idea is stated most clearly