I don't think this is really a reflection of any sort of quality of new music vs. old music, but rather the structure of how people listen to music. If you listen from a main playlist on Spotify and add 100 songs a year and you listen to it generally on shuffle, you'll progressively listen to less and less new music as a proportion of your total listening. This is despite you being interested in the same amount of new music each year. This seems like a likely culprit particularly considering the 18-month limit this infograph seems to consider as new music.

In ye olden times one had to actively choose music to listen to, lending itself to the newer albums, and a relatively weakened aggregate power of one's back catalogue. Spotify and similar services lend themselves to a very passive means of listening however (shuffle and go) that will naturally give more listening to the music which has greater quantity (old music of course).