Yes, online could have probably been put in. However, they chose not to, probably to meet their deadline.
It's one thing to say, "Damn, I want to play this online, I wish Nintendo put this in." It's another to say, "This is bullshit, online is a standard in 2009!" Local multiplayer has been around since like, Super Mario Kart, and has been "standard" since... the N64? However, practically nobody pays any attention to it nowadays, and I rarely see this "industry standard" bullshit brought down on devs when they give local multiplayer the shaft.
So, the real reason for the bitching is not because online is an "industry standard" - it's merely because the internet cares about it more than local multiplayer. Sorry to those who wanted online play on NSMB Wii (even though we knew from the start it wasn't there), but us local multiplayer fans aren't given nearly as much attention, and after this fiasco the issue has really come to light. Online gamers have two entire platforms (PS3/360) catering to them. I'm all for fighting for online play (and calling out laziness) regarding NSMB Wii... as long as you're willing to do the same for split screen online in the next Modern Warfare.
