This article sets up a strawman argument by asking (and answering): “Are all johns so unappealing that they cannot get sex any other way? Most researchers do not think so.”
Of course, there are probably many different reasons men pay for sex. But “being able to get it another way” doesn’t mean they won’t resort to a prostitute, for a very simple reason: in any relationship, you cannot always have sex on demand, can you? And the willingness of your partner to fulfill your fantasies has some real (if variable) limits.
Not so if the requirements are negotiated and determined with a price point.
(Full disclosure: I’ve never paid to have sex. The limits of traditional sexual relations have never been onerous to me.)