
There are plenty of other reasons which I've never seen discussed:
The ability to make vast sums of money from selling CDs may have been an historic anomaly. For a few years it was possible to duplicate sound production for almost no marginal cost but sell it at vast profits. The industry has grown fat on this, but now the same means of production is within the grasp of most people, it has become a tough sell!
Visit a any shop, what do you hear? Music at zero cost.
Visit a gym, what do you hear? Music at zero cost.
Get in your car, turn on the stereo radio, what do you hear? Music at zero cost.
Go home, turn on the hifi radio, what do you hear? Music at zero cost.
People are used to getting free music - hearing a song on the radio for free but being told it will cost GBP15 for the album if you want to replay it is jarring. Getting music for free (from the internet) is more in line with what we are used to. Perhaps if the music industry didn't license out its music, it would be more scarce, valued more highly and purchased by consumers more.
As "popular" music stations proliferate, maybe people don't feel the need to buy music when you are bound to be able to get it on some station or other.
We know the CDs cost almost nothing to make. We know singing costs nothing (we do that in the shower), so why the big cost? Perhaps the dirty secret the industry cannot admit to (though everyone knows it) is that music isn't popular unless accompanied by vast advertising budgets. Like US elections, it isn't the quality that is important, its how much you spend promoting it.
Perhaps the videos used to promote the music is as extreme as the industry expects to be able to push at young people without causing an outcry. Having pushed ever more stimulation at its target audience, the audience is now bored and moved onto hard core porn, which it gets free from the, er, internet.
If your business is making money from singers singing they may actually have to do that. Few "normal" people get paid for years for work they did in the past, so they have little sympathy for those complaining that they aren't getting any more money for singing something last year. This is especially true when the complainers are seen to be multi-millionaires.