The declining revenue issue is a pretty easy item to figure out:
If you think about it, iTunes is the 1-stop-shop for all the music you would ever want to buy (that's the sales pitch but I still don't buy from iTunes...ever.). So you have this gigantic store with hundreds of thousands of songs. iTunes opens for the first day and as the months and years go on, millions of people buy music because of the inexpensiveness and the sheer catalog. People are NOT buying new music (necessarily), they are buying a new medium (files vs. physically something in your hand).
Now fast forward about 8 years when, gulp, everyone WHO WANTS TO OWN a digital music file basically has...all those wonderful tunes from the 50s to today are owned. People are not going to go back and buy the same items again. Now your customers must buy from the PRESENT...the songs that are good TODAY. The customers are only now buying a few songs/albums a year because, well, you guessed it (and I said it a few sentences back), they already own all the other wonderful hits. And unless you release a new medium/format, all these customers are not going to re-buy anything.
What I have just described is exactly what has been happening to the music industry AS A WHOLE starting in the late 90s...vinyl is the medium in the 70s and everyone buys it...then cassettes come along and everyone kind of restocks their home catalog with cassettes (that not only provide a smaller size but also ability to be used in cars, boomboxes, etc)...then cds come along and everyone re-buys older stuff on cd because of improved sound quality, extreme durability, and other reasons...then from the late 90s to the mid 2000s the music companies notice that gee, nobody's going out and buying all that "back catalog" music anymore...and the industry blames mp3s/pirating/cd copying.
So here we are, iTunes...you've made a lot of money selling billions of songs. But now everybody owns all the older material. So iTunes "sales" begin to decline (around late 2011) because again, everyone already owns it all. The only way iTunes would sell more and more music (meaning, the same old songs) is if the songs were re-released in another format...such as Lossless. But Apple doesn't want to do this...or if it does, it's all wrapped up in either DRM or some proprietary Apple format that can't be used/converted to anything industry standard to play on a computer or transfer back to a physical cd.
What I have described will ALWAYS be the case with selling music...either there needs to be a new format every 5-10 years to keep people re-buying their old/older tunes or there has to ALWAYS be an extremely high quantity of present-day music that will keep people coming back every Tuesday of the week (that is the day new music is released in the USA).
iTunes is now choosing to expand into streaming...which is fine. But I'm completely uninterested in PAYING for music that I can't physically own. I am not a believer (or "truster") of the cloud and similar "services".