I think the point people are trying to get at is that online does not nesscessarily beat non-online.  There are some games where online is absolutley critical (Battlefield Series, Shadowrun), but games that the online is crucial usually lack in the single-player department. 

Single player and multiplayer experiences are totally different.  If a series that previously had an exceptional single player switches to a lackluster single player with online multiplayer, the experienced is ruined, and I'd rather have the single player way. 

Let's say if for GTA IV, Rockstar decides to implement an online deathmatch part, but in the process, the core game gets slashed down to a small portion of what San Andreas was .  I know I'd much prefer the full single player game without any online multiplayer.  But maybe that's just me.