Writing isn't alone for this one. Musicians, actors, and artists fall in the same group. These are things a huge number of humans are capable of doing at a very high level. Many perform irregardless of earning potential.

My guess is that the long term earnings for writing will move closer to zero. The best writers working for the best organizations will still be able to make a lot of money.

We have just barely started moving in to a period where online education turns billions of uneducated third world/developing country citizens in to highly educated English speakers.