>>74707687natural fermentation will not go above the limit of the yeast, which is typically 12-18%. any remaining sugar will not be processed and is effectively preserved by the alcohol. the resulting wine also is not "oxidized" to create vinegar, but is fermented by bacteria in the presence of oxygen (it digests the alcohol into acid, not the sugar). as long as you limit the amount of air in the container, this will not happen.
what you don't understand is that the vinegar is sweet due to sugar that was present in the wine. it is there because there was too much for the yeast to process, resulting in the max amount of alcohol as well as the remaining sugar. they achieve this by reducing juice, which does not involve adding sugar but still greatly increases the concentration of it. this juice (now wine) is so sweet that it retains a high amount of sugar after both processes, which is to say, even once it is vinegar, it is still very sugary.
sugar turns into alcohol, but this statement is far from absolute and i am providing you a very clear example of that not happening. it begins as sugar, and even after being fermented, sugar remains. stop being fucking autistic.