The switch to sugar alternatives is already happening.

https://www.marketingweek.com/diet-coke-sales-overtake- classic-coca-cola/

"Compare this to the week before, when Diet Coke sales were at £10.4m and classic Coca-Cola £10.65m. The data shows Coca-Cola value sales were higher than Diet Coke every week for the past year until the brand overhaul and subsequent introduction of the sugar tax. Since then they have diverged with Diet Coke sales increasing at a faster rate than classic Coke." [2018]