Like, would it taste that much worse if they put less sugar in it?
It actually tastes better. Coke has something like 80 grams of sugar per bottle. Natural/organic/specialty sodas all hover around the 20-50 range. In the absence of sugar you can actually taste the other flavor compounds present in the drink.
Also I usually can't taste much of a difference between real sugar and HFCS except for two sodas: Sierra Mist and Malta Goya. Malta Goya also has a better mouth-feel when sugar is used. Sierra Mist I'm actually less sure if the difference is the sugar, because Sierra Mist Natural uses an entirely different formula from regular Sierra Mist anyway. I don't think they make the original Sierra Mist anymore anyway -- thank god -- so I can't do a direct comparison (not that I'd want to).
Although come to think of it, I feel like natural ALSO has a better mouth-feel. I feel like HFCS gives stuff a SLIGHTLY oily, very syrupy texture.
BTW, one thing with HFCS -- having tasted corn syrup on its own, it's sweet, but doesn't really have any flavor besides being sweet. Same for sucrose. Natural or raw sugar still has some amount of molasses in it and generally has a much nicer flavor in and of itself. Soda mostly uses HFCS or crystalline sucrose, so neither are going to bring a lot of flavor to the party. It's more like the HFCS keeps stuff from mixing properly.