> Why would a reduction in human writers cause a complete reduction in motivation to read?

Because there would not be human written books about the present. All books would be about the past. But literature is not stuck in time. Today writers talk about topics and feelings that writers of the last century might never know or experienced. Many people read books to better understand the today world (non-fiction) and to better understand their today feelings (fiction).

> People write for hundreds of reasons other than to make money.

Agree, but most of the writing that we have from the past still came with some form of financial incentives. Shakespeare didn't write all of the compositions just because he wanted to express himself. He was making money with theater performances. Many religious writing got patronage by the church. Dante Alighieri had a career as politician, Plato came from an aristocratic family. Writing was reserved to elites because education was expensive and people had to work for food.

Today we are lucky because education is accessible and printing is cheap.

> they created literature before copyright was a thing.

Copyright wasn't a thing because replicating content was hard. Try to manually copy a book...