Reselling does nothing for Sony though, so losing the customers who are buying and selling used games isn't a loss for them.

It's like how people freaked out about how Netflix would surely collapse when they cracked down on password sharing, but it's actually brought in a bunch more money for them. Losing viewers who aren't paying for the service was actually a bonus. Even if there had been a bet zero change in subscribers, they'd save money from the reduction in traffic.