That's almost what happened, FOX entertainment chairman Kevin Reilly told reporters at this year's TCA press tour. Back when he was at NBC, Reilly bought the script from original Walking Dead showrunner Frank Darabont. "I developed it. I loved it," Reilly said (via Digital Spy). "[But] I was absolutely at the bottom of the ratings at NBC, and I just was like, 'Really, Frank? A zombie thing?' — I thought it was good, but it was an early draft. And then, when I left and I heard it went over to AMC, there was just a lot of serendipity involved."
After the session, Reilly told Deadline that if TWD had aired on NBC, it "would have probably been a little less intense and a little less interesting to the audience." It would’ve been a lot less violent, too, although Reilly said his FOX dramas, like The Following, have to compete with the likes of TWD so they are trying to "match the intensity, otherwise we'll pale in comparison."
NBC is probably kicking itself that it didn't keep TWD, but it would've been a much weaker show, so we're glad they ultimately passed. Then again, what would TWD be like if, say, HBO had picked it up?