There's literal pennies of sugar in the typical product. They aren't pushing sucralose to get the extra bit of margin, they are responding to the large number of people that want lower calorie sweets.
I don't think the latter factor, which I mentioned, obviates the issue of cost. The cost of any ingredient is going to be "literally pennies" but that isn't an argument that it doesn't matter to the manufacturer nor that they don't try to optimize it.