I’m with you. If I was making a Port-style wine and it was 20% alcohol and 100–150 g/L residual sugar (10–15%) I would forgo the potassium sorbate altogether. I am not a fan of potassium sorbate because I think it makes wines taste funny and seems to be a holdover from an earlier era when home winemakers couldn’t get good filters and when it was added to everything as a refermentation retardant. Additionally, the elevated alcohol and sugar both will work together to retard most yeast or bacteria that may cause a re-fermentation in the bottle.
Just make sure that indeed your wine has these kinds of statistics for robust aging. There’s nothing worse than what I might consider a “weak” Port-style wine. In fact, it’s probably one of the reasons I dislike so many California Zinfandel table wines. They often are 16% alcohol and 5% sugar and can’t seem to decide if they want to be red wine or Port. I prefer to go one way or the other for both style, palatability, and stability reasons. So if you’re over 20% alcohol and 10% residual sugar I would say you could dispense with the potassium sorbate.
