Digital artwork is easily pirated, paying is optional.
Optional payment is a gift, a reciprocation, a thanks, honouring its creator.
We give to reciprocate, so why give most of the price to intermediaries?
I have come to realise that the above are all unavoidable statements, in order of descending certainty.
In a traditional thing-based society, we can use physical force to make sure people pay for what they take. This is necessary both because taking means the original owner no longer has it (copying is manufacture), and because resource is limited. But in a digital world, I can give you a song and keep it as well. It is also near-impossible to enforce (literatally, force) laws controlling sharing and copying of digital media.
The response of traditional media and the status-quo has been to attempt to enforce thing-world control of digital artefacts; creating rights management software to prevent you distributing your music by making it more like a physical thing; traceable, hard to copy, sending the police to find you if you disobey. Understandable, but a losing battle. Computers change often making any such systems obsolete quickly (Why can't I play my albums any more? I paid for them!) and widely evaded, because imperfections are quickly exploited and the exploits easily distributed. The internet is made to distribute data quickly and freely, so all control systems are fighting the basic nature of the system.
The result has been a two-layer system: On one it is easy and free to get digital artefacts without paying for them; until iTunes and Spotify, it was actually easier to pirate music than to go and buy it. Cheaper, faster and better = revolution. On the other it is easy and reasonably cheap in moderate quantities to buy or license music. Most people, particularly the teenagers who will drive the future culture, therefore have a choice whether or not to pay.
It is not quite as simple as clicking Pay or Free download; for the older generations (twenties and up) there is a significant social distinction between pirating and buying, and the legal difference means Spotify is limited to legally licensed music. So the two layers are quite separate, meaning each group tends to employ only one or other system in the main; some groups download everything, some buy everything. The difference is predictably related to disposable income as well as socially-enforced stigma.
It is, however, already a reasonably free choice for most people whether to accept a minor amount of guilt and download something to try it out, or to stump up for it. The barrier is far lower on renting sites like Spotify, but it still exists. I can opt to look for an illegal electronic version of a new book or buy the hardback; buying the artefact is seen as a way of supporting the author, in the same way that ticket and merchandise sales are seen as a way of supporting a team.
So people are already used to making a morally-guided choice based on personal expression about where their money goes; we have been taught by consumerism "You are what you buy". If you doubt that for a moment, consider whether any advert for a premium brand focuses on the product or the viewer's self-perception. You pay a bit more because you're worth it.
If I buy a hardback book, I know that most of the money is going to the printer, the publisher, the retailer. But some of it will go to the author (once past their advance). We accept this bargain; this is the author-sanctioned mechanism for both thanking the author and obtaining a physical artefact of self-identity; "Thank you Jo Rowling, my copy of The Half-Blood Prince will take pride of place on my bookshelf. I am a Potter fan."
But if I pay for a copy of an ebook, there is no printer, no retailer. If I already like the author, they have no need for a publicist in this transaction. It's me thanking the author and their editors. Why should iTunes or Amazon take a cut?
The (small) cut should go only to whoever creates the system permitting me to thank the artist at my discretion, which somehow lets me build my identity by the same token.
This kind of giving is expressive, social and efficient by minimising middle-men. Just what the internet is for.