Today, we are back to the style of selling music like it was throughout most of the 1950's--music sales by individual song.
In my opinion, what may finally put an end of Compact Disc sales is when online retailers start offering their music in lossless formats such as Apple Lossless, FLAC or possibly WMA Lossless formats. With improving Internet bandwidth and cheap server storage nowadays, we could just see that happen within the next 3-4 years. The most likely lossless format to become widely commercially available for download will be Apple Lossless, since the iPod nano, iPod classic, iPod touch and iPhone support the format and the higher end players from Creative, Microsoft and Sandisk could upgrade to play the Apple Lossless format with a firmware upgrade.
Exactly, except sales by individual song goes back to Tin Pan Alley days and the popularity of sheet music. The emphasis on selling only albums is a comparatively recent development.