Do we actually know this, for sure? Obviously, it seems like common sense, but there's a similar argument about piracy, and an opposing position that piracy never managed to kill the industries it was apparently a threat to — if anything (so the argument goes), it might have even boosted them. I've bought physical books in the past, even though they were open source. I might also spend money on a digital open source book for various reasons, some of which you mention. It's likely that an author won't make as much direct income from an open-sourced book, but they might make 'enough', even before you consider indirect income.