Diablo has always had a strong online component. While the series has never been an MMO in a strict sense, online play has always been more engaging (except for rampant spamming... which Blizzard seriously needs to address). Sites like d2jsp.org will attest to the added depth of online play. An economy developed out of the items you could find and there was also high end game play that was only available online. The experience was clearly superior.

Lots of people are going to jump all over Blizzard for it, but this move was entirely predictable and makes sense. What do you gain by playing offline: less restrictions and less gameplay. While some people will disagree with me, it is akin to asking for an offline component to WoW or EVE: there's technically a game left, but without other people there's much less fun and enjoyment.