What I imagine happening is people making a big stink about it being online only, buying it anyway, then still complaining that it's online only.
I love how people buy a product, then complain on the forums about a feature they knew was there but don't like. As if posting poorly thought out complaints about something negates the fact that they supported the company in pretty much the only way that matters: buying their shit.
I don't much care for the idea of single-player games requiring the internet, but the future is now and there's also lot of good ideas being pushed around in the gaming world. If a company dedicates itself to supporting a game long after it's release (as Blizzard is famous for), I can forgive online only. I'll play into your 'online only' crap, you make sure my game is perfect before you stop working on it, deal?