Of course, it is impossible to "prove". But there is a strong correlation between the rise of downloading and the collapse of sales in the music industry. It would be sloppy reasoning to dismiss any causal relationship between the two a priori.
Please be courteous and refrain from using obvious straw men in your arguments. It makes the person you are responding to feel like you are not taking them seriously.
I have often wondered if people buy less music because they can try it first and realize it's not worth buying. I can't count the number of times I have been excited about a new album, downloaded it when it leaks online, and realized it sucks. Before, I would have been excited, bought the album when it came out, realized it sucked, and regretted my purchase. Now I can save myself the money.
Of course, the reason could be as simple as attention spans. People only have so much time to be entertained (at most 24 hours a day)... and there seem to be more movies, tv shows, songs, video games, websites, etc than ever. If people are spending more time watching movies and playing games, they are probably spending less time listening to music... and hence less money buying new music to fill that time.