I'm pretty sure the causality is backwards. People aren't drinking less because they hear people saying it's poison, people are more willing to tolerate people calling it poison because they're drinking less. People have preached about the evils of drinking to excess as far back as we have records of people discussing stuff. It just got ignored, because it's a crucial social lubricant, and thus everybody imbibes anyway. With the rise of addictive social media and gaming, we've seen a decrease in socialization in meatspace, and that's where people drink. If you're missing out on the positive aspects of alcohol anyway, then it's a lot easier to listen to people talking about the negative aspects. And then those voices get amplified to justify not doing the thing you weren't doing anyway.