Ok, I'll bite on this and admit that you have at least the bones of a reasonable argument there, and that a couple of your points are valid. However, I don't believe that taken as a whole, they amount to an argument against Diablo 3 needing an offline mode.
Over on the consoles, games that actually require online connectivity to play are few and far between. There are certainly games that lose a good bit of functionality if there's no internet connection present; Gran Turismo 5 on the PS3 and Your Shape 2012 on the 360/Kinect stand out as good examples. But the core gameplay is at least accessible offline. There are a couple of exceptions - mainly a few Capcom downloadable games on the PS3. Don't buy them. Certainly not after last summer's PSN outage made them unplayable for months.
Similarly, all of the iPad games that I have been willing to pay money for are playable offline. There are others that don't meet that criterion - and I don't buy them. Facebook gaming? Feels like a step back to me, not progress.
I don't think I've ever argued that all PC games must be playable offline. I excluded MMOs in my earlier post because the very nature of the game requires an always online connection. I suppose I could have excluded multiplayer-online FPSes as well, as they fall into the same category, but I don't buy those anyway (not for ethical reasons, but rather because deathmatch as a game-mode hasn't really appealed to me for 7 or 8 years now). In fact, even with those, I'd expect an offline bots-mode, as it's fairly trivial to implement.
But if a game is to require an always-on connection, then it needs to have features which are both essential to the gameplay and of benefit to the player. And Diablo 3 fails on both counts here. The big thing in Diablo 3 is the auction house - which has in-game currency and real-cash variants. The in-game currency auction house is of some benefit to the average player, but nothing I've seen thus far suggests it's even close to being essential for play. The real-money auction house is frankly only likely to be of benefit to a small hardcore and to Blizzard's coffers.
And I'd dispute that Diablo 2 was "miserable" offline. I moved house last month and spent about 5 days without a home internet connection while I waited for my ISP to hook my cable up. I used some of that time (when I wasn't unpacking boxes) to replay Diablo 2, to remind myself of the plot. The game hasn't aged all that well in some respects, but it was far from miserable. Certainly, it was more enjoyable than the 20 minutes I have just now spent copy-pasting my password over and over as I tried to login to Diablo 3's servers so I could play a singleplayer game.