Have you ever seen a sheep be sheared? It's violent and bloody. If your barber scraped the hell out of your scalp while shaving your head, you'd fire them.
Also, sheep too old to produce good wool don't get a peaceful retirement. They get slaughtered and turned into dog or chicken food. The same thing happens when there's a disease epidemic - common because of the crowded and filthy conditions in factory farming - or crop failures or drought. As soon as it's not profitable to keep the sheep alive we kill them.
But neither of those points are actually the point of the conversation at all. The point is it's immoral to use an animal as an object to benefit humans. If you wouldn't keep humans in pens and shave them to make clothing, you shouldn't do the same thing to sheep. Simple as.