There are other digital services, but itunes is used by far the most. Some people would buy more stuff off other outlets for a week, but a lot wouldn't, meaning a significant decline in singles sales. This would therefore alter the (currently one sided) balance between digital and physical sales, leading potentially to a chart with a lot more indie music in it, and also a lot less back catalogue stuff that pretty much clogs it up these days.

It would be interesting certainly, though how big the difference would be would be something hard to predict until it actually happened i suppose.