Observations of fellow readers, conversations with self- and traditionally-published authors, and some knowledge of the market?
But what is low, anyway? For the sake of argument I could believe 10, 20, even 30% of all the books people read are pirated. I would be surprised if it was higher, but let's just say hypothetically it's 50%. I think that's a reasonable conservative estimate. So, in this scenario, the remaining 50% of reads can in principle be monetized by their respective authors.
Abolition of copyright will drive that monetizable share essentially to 0%, for reasons I've outlined elsewhere in this thread.^[1]. I consider that meaningful, and I have personally had conversations with published authors who state that the royalties they receive are financially significant, which is why I'm here in this thread taking the position that I'm taking.