When someone buy a song, there's no assurance that he'll play it again. He might play it once or twice and stop doing it after and just leave the song on his library. Many ppl also buy songs that they don't like, maybe just bc it's popular or owned by hot artist so they bought it out of curiosity.
But with streaming, you can tell that it's either new ppl are streaming the song or the same ppl are playing it over and over again bc they like it. A song won't reach hundred of million s of stream if it's bad, right? Those songs that got popular on its first days just bc of pre- release hype tend to lose a lot of streams on the following week or month bc ppl realize it's a bad song. Also, putting a song on popular playlists doesn't mean ppl who followed the playlist will play it. I follow TTH playlist on Spotify but i only play 7 out of 50 songs inside it. There are more reasons why it's more accurate indicator than sales but I'll just leave it here.
Anyway, do you agree?
) and streaming is taking over. 
