I thought the main advantage of the record companies was that they could loan artists large amounts of money to make and market a commercial record.
Most musicians I know are poor and would make pretty poor business people. Unless spotify or apple decide to go into developing artists themselves.
Because streaming services wouldn't own rights to artists' work and they wouldn't be tying them down with multirecord deals. With these constraints lifted, there'd only be free market mechanisms left, which would force the services to be nice both to artists and users.
What's to say streaming services wouldn't run up the price on artists once record labels are out of the picture? You could argue that artists could simply take their music to another service but if one service has A LOT of users (ie, Spotify) that might not be a viable option.