1) Say they're wrong. Argue the facts. Point out that historically speaking, similar predictions or theories have been presented by others and none of them have proven to be correct.
2) Point out that in an organization the size of Google, even an organization like Google that hires very academic and smart people, only a percentage of them care to debate a particular set of views. Another (larger) percentage are just going to write you off because they don't have the time or the inclination to discuss that thing; they have too many other things on their plate. So you're kind of a moron for antagonizing xx% of the people you need to work with on a daily basis. You're deliberately breaking the "No Brilliant Jerks" rule. So the professional consequences should be perfectly predictable before you ever opened your mouth, and no one is at fault but you for getting fired.