"There is no such thing as a >50% attack of Peercoin's PoS process."
You are assuming there is an attacker or an attacker who wants to benefit directly from double spending. It could be an accident. Say Peercoin becomes a main stream currency and there are banks or wealth management companies of cryptocurrency that offer real, attractive interest (via investment etc.) to depositors. Some of these financial entities could accumulate more than 51% PPCs, just like the aggregation of hash power to big pools in BTC’s POW network. Then accidental double spending could happen (check out blockchain.com. there is a record of suspected double spending events that shows it happens all the time). Or it could happen because a competitor/vandalist has broken into the servers trying to ruine the financial entity.
Although such double spending could be an accident/crime, it would still be a real possibility. People’s confidence will still take a hit nonetheless.