1) I don't see how the Google Trends line proves anything; it certainly doesn't say anything new.
2) Correlation does not necessarily prove causation. There is no particular reason to presume that "going DRM-free" was solely (or even predominantly) responsible for the increase in iTunes music sales. E.g. promotion was minor; and the number of iPods sold went from 28 million by the end of 2005, to 226 million by the end of 2008. I'm gonna guess that was
one factor (among many) for an increase in song sales volume.
3) eMusic has sold DRM-free MP3's for years and years. Care to guess what their sales are like?
4) DRM in various forms is still a huge component of digital content, even materials distributed by Apple. E.g. the billion-plus apps sold for the iPhone are all restricted; iTunes videos are still restricted; video games, DVD's, Blu-Ray, computer applications and so forth, none of that has changed.
5) Music has a significant issue with digital distribution, namely they released their entire catalog for years in an unprotected digital medium (CD's); so it's a snap to rip an album and share it. The horse was out the gate and long gone by the time they started selling DRM-free tracks. Books don't quite have that issue.
Despite the numerous similarities between ebook and digital music sales, I do not see any indication -- and nothing new -- that "DRM seems to be losing."