yeah, sarah, i used to think the same thing until i ran out of my usual mixer and tried plain soda water with just a splash of lemon juice. it was honestly fine. not amazing, but totally drinkable. what worked for me was keeping a bottle of that concentrated no-sugar cordial from the grocery store, the one that's like 2 bucks. just a little squirt and it's basically a fancy soda. saved me from going out to buy something.
Just buy a soda stream and mix it with some cheap squash. I've been doing that for years and it costs pennies. Save the fancy syrups for guests or whatever. Plain fizzy water with a splash of lime cordial is basically a gin and tonic without the gin. Not exactly a tragedy.