Neobr10: But that's pure speculation. There is no way to know for sure if sales were higher or lower with always-online DRM. The things that we do know is that it sold 12 million copies (which is quite a lot for a PC exclusive) and that pirated copies don't work.
However, as another poster mentioned, we also know that Diablo 2 sold 17 million (and the expansion sold another 17.5 million). While it's not necessarily fair to compare a game that's been out for 13 years with one that's only been out for a bit over a year and a half, the sales for Diablo 3 seem to be dropping off rapidly (for the numbers I could find the sales in 2013 are currently only 10% of the sales in 2012). Not to mention that the size of the potential customer base is significantly larger now compared to when D2 was released (plus D3 had the momentum of D2 driving its initial sales). What will be somewhat telling is looking at the sales numbers for the expansion once it comes out, to see if D3 as it currently stands looks to have anywhere near the kind of momentum that D2 had.
Of course, the difficult is teasing out what effects the online-only aspect of the game has on all of this vs the impact that the rest of the game's design decision had. But it should also be kept in mind that the online-only decision was linked with other design decisions (auction house, and the resulting tuning of certain game mechanics, so it's not as if these variables exist completely independent of each other).
What would be quite telling is if Blizzard drops the online-only requirement in the near future, as that would pretty much be an admission from them that is wasn't having the effect they wanted. So I'll be watching with a bit of interest in the months following the release of the expansion (yes, I know Blizzard has stated they won't be removing the requirement, but I'll let you guess how much stock I put in words from their PR department).
