That’s too absolute: nobody is arguing that there’d be a 100% purchase rate but it seems highly unlikely that there will be no sales impact from having free copies available from a reputable source with very high search-engine ranking.
>> "nobody is arguing that there’d be a 100% purchase rate"
The typical case against things like this does multiply downloads by sale price under the assumption each one is a lost sale. I've seen authors (obscure and prominent) scream about how many hundreds of thousands of dollars they're missing out on because they found their book on some torrent site.