If you SUBSTITUTE malt for sugar, then final gravity will decrease. This is because you've gone from something 65-75% fermentable to 100% fermentable.
But if you were to ADD sugar to an existing recipe, the result goes more like this:
Adding sugar increases the original gravity, but the final gravity will most likely stay the same, or perhaps will decrease by like 1-2 points, maybe, but not that anyone would really notice. For example:
You have a 1.060 original gravity beer, with an expected final gravity of 1.015. If you ADD a pound of sugar, your original gravity might go up to 1.070, something like that, while your final gravity stays the same or maybe goes to 1.014.
The result is... more alcohol, with zero impact on the flavor except the greater alcohol. You've gone from a 6% ABV beer to a 7% ABV beer. The body and mouthfeel might feel slightly thinner but most likely won't be impacted very much at all.