I think it's simpler than that. Most people don't have a clue about DRM. What they know is if they want to listen to music they go buy an iPod and buy music on iTunes. It's more about brand recognition IMO.
They may not have a clue about DRM, but what they do understand is, "If I switch to <competing thing>, I won't be able to use it to listen to my existing music collection, or read any of the books I already bought, or run any of the apps I downloaded?"
The iPod isn't as dominant as ever, iTunes is only partially DRM-free (many songs still aren't available for sale that way), existing songs in your collection don't get unlocked, and users have to either pay per song or subscribe to iTunes Match to unlock them.
And that is all caused by DRM.