Online or single player only, the larger, abstract question is what draws players to buy your game? Is it the epicly produced, long waited for single player experience to die for as Final Fantasy 7 was? Is it the game that starts out as a friend pass, but comes to consume your life like the World of Warcraft? Is it the competitive online experience where burger flippers can pwn CEOs in Call of Duty: Black Ops?
I don't think gaming has reached a point yet where all games from December 8, 2010 onward all have to have an online option otherwise they won't sell. I don't see it in wake of Fallout: New Vegas selling comfortable numbers.
For 2 years straight I played nothing but the World of Warcraft. During that time I bounced from guild to guild, changed servers and only started to seriously play with others to raid my last 8 months playing the game. Well over half my time playing the World of Warcraft was a single player experience punctuated with me screaming at my own team in PvP battlefields and being one of cool guys with my guild during a raid where I got all the loot I wanted because I was the only hunter amongst 6 death knights, 1 tank (usually a paladin), and 2 healers (druid and priest)
What people forget when they hail online gaming is many, including myself, at times will play it as if they are playing a single player game. When I log on to play a round in Bad Company 2, I do not wait 30 minutes much less 1 minute to find a friend or two to play with. Hell, I don't even play for them, I play so I can wrack up some kills and enjoy the quick action. When I played FarmVille, it was a selfish desire to create my perfect farm and I only helped my friend's farms because I was rewarded for it. Take out the reward and I would never vist their farms.
If you think online option is the future, then you have to expect and accomodate for a sizeable number of your playerbase to play it as if it was a single player experience.
