It sucks because Diablo 1 and 2 supported local play but if Diablo 3 were the first in the series I don't think anyone would be complaining.
This is a step down in features for the consumer (just like the step down in features for Starcraft 2 from 1). There's no reason to paint a rosy picture about it. Last I checked there were still more active custom Warcraft 3 games than SC2 games and that speaks volumes.
You honestly think they said "hmm, let's save money by requiring online to play so we can better serve the customer." or do you think they said "let's make more money by further ensuring the integrity of our real money auction house and giving us more control over the product."
Hell, my favourite mod is still being actively developed: http://modsbylaz.hugelaser.com/
I have to disagree. Diablo 2's robust player economy was the driving force behind the game's lasting success (hence the advent of cash auctions in Diablo 3).