First off, for someone not interested in Diablo 3, you seem to be keeping a tight eye on the fansites. You sir, need a new hobby =)
Next let's examine the fallacies and false assumptions in your post =)
Why would this forum contain the oldest/best/most faithful followers? I'd personally say the old style battle.net forums are at the very least a very good match.
Also, where on earth are you getting this "a significant portion of the fanbase" from? The ones posting on any forum are but a small minority of their total customers, and if you pay attention only a handfull of, admittedly very vocal, people are screaming bloody murder about the games development and have done so since pretty much the beginning. This is the same for the development of every game. All the rest is simply anxious for a release.
Now to adress your concerns about offline/online play. How is this different from a game like WoW? Atleast with Diablo 3 they have the possibility to patch in all the server side activities to be handled on our own pc's, making it offline play, should that dark dark day ever come. Reasons why they wouldn't do this are basically limited to; armageddon, gouverments collapsing into anarchy, or a world disaster whiping out civilization as we know it. In all these cases, I wouldn't be all that worried about the fact I can't play Diablo 3 anymore.
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