Yaez, I agree with most of your points, but Diablo 1's ending (and therefore the plot of 2) kind of relied on playing solo, so it was more of a single-player game with a multiplayer option. So, no, it wasn't always a multiplayer franchise. Besides, what's the point of having a single player option if you have to be online all the time in order to play at all? Oh, and I don't think that not having always-online DRM is the world "staying still", it's a matter of not bothering with DRM which will only serve to inconvinience legitimate customers.
I realize. But even back then, Diablo played much better as a multiplayer experience. The original Diablo didn't even have hirelings to help make up for your PC's lacking traits. And the completely random of drops really does just lead to a multiplayer experience being better, where even if an item you find sucks for you, it may be awesome for someone else.
As for single-player being online, I do find a rather appropriate reason for it, and that's eliminating the separation of single and multi-player characters. In Diablo 2, you couldn't use your single player character online, because there was no way to ensure the character wasn't altered using third-party programs. In Diablo 3, since this separation no longer exists, one can play Normal difficulty solo to experience the story at their own pace, then go multiplayer with friends for Nightmare and beyond, using the same character they've already been working on. When I do finally get the game, this is the route I'll likely be going myself, and I'm really glad I have that option. It's an option I wouldn't have if Diablo 3 weren't always-online.
I do realize it may be an inconvenience for some. But always-online features are what pretty much all games are eventually going to have, even console titles. Physical medium isn't going to be standard for much longer, and one could reasonably make the case that it's already archaic--many high-end gaming PCs don't even come with optical drives anymore, because they're often pointless and are wasted money. The future sucks for people without broadband, but at the same time, that accounts for fewer and fewer people every day. Game companies can't make progress if they ignore the progress the rest of the world is making.