While certainly an option, just watering out a drink more isn't really what they're asking for. There's lots of other flavor compounds that you don't necessarily want watered out just because you want less sugar.
Diluted juice is still a useful suggestion as an alternative idea. I don’t get why people would down vote; that’s not what down vote is for. I like diluted juice quite a bit, 25-50% tastes fine, it’s akin to tea to me. It would be interesting if more beverage companies made a “add your own sweetener” version which is how unsweetened Kool-Aid drink mixes come. They have maltodextrin in them but say they are 0 calorie so I’m not sure if that means it’s a tiny amount or if it somehow gets away claiming zero calories. Just be aware Kool-Aid has sweetened versions as well. I still mostly drink water but some variety is nice.
Oh this is where I'm at now, I've been buying juice and adding a little to soda water so it comes out like a light Fresca sort of drink. It would be nice to have that option when going out though.
Oh I definitely agree it's a good practical suggestion, I do it with apple juice sometimes because I just want some mild apple flavor rather than the very strong and sweet apple juice taste. I don't get why someone would downvote it either.