They should rather just drink water.
Since cutting down on sugar I almost never buy soft drink. These days my intake is water, coffee (1 or 2 at most a day), tea and Woolies milk (unfortunately not Raw).
Drinking sugar less soft drinks are unfortunately just as bad.
Sugar tax is not there to bring the usage down but to rape more citizens. Sugar can be addictive and used in allot of products, so it is just an easy way for government to get more pocket money and to make citizens spend more. Will the price of sigarettes make a smoker stop smoking, no. They call it "sin tax" because it makes the corrupt government sin and the citizens pay the price.