Yeah, that's kind of why I said the printing press is too nebulous--there's nothing physical (like phone lines or running water) to prevent magic. Or, like you said, extra pollution in the air, which would work perfectly well. I think I'm leaning more and more toward the use of fossil fuels...

I don't know about "purposely" though. You're saying that it would be more believable to you if, for example, people started to use fossil fuels precisely because they thought it would make magic more difficult?