However, if it wasn't in the interests of content providers to provide ways to buy their content outside of iTunes, they wouldn't have done it in the first place; so as far as many content providers were concerned, the pros of not going through iTunes outweighed the cons of the risk to the iTunes channel itself.

Well honestly.  100% profit versus 70%?  I'm going to choose 100% all the time because that benefits me, the manu, the most.  As a manufacturer I might not care so much about the longevity of the channels I use, because the product I sell might not have that much longevity or its outlived several channels.  The channel itself however will want to remain viable as long as possible.