Have online streaming services like Spotify reduced our willingness to pay for music?

Services like Spotify and Youtube are paid for by adverts, so it's not exactly true that people are not paying for them - the end user doesn't pay, but the advertiser does. It's a similar model like a commercial TV channel. But yes, end-users aren't buying singles or albums very much any more. Some artists are even announcing that they are releasing a "playlist", not an album.