It really wouldn’t. If a corporation reduced their production of for example red meat, another one would simply scale up their production, because the demand of the market would remain unchanged.

Also, there’s already more than just a “small percentage” of people who have dropped red meat from their diet. All vegetarians, vegans, pescetarians, and people who eat meat but stopped eating red meat due to the environmental impact add up to several percentage points, which is absolutely measureable and impactful.