I support Eric here. Singleton make sense in WordPress specifically for encapsulating plugins. In WordPress plugins can never have more than one instance so it's really a moot point to expect them to be anything else.

Outside of WordPress plugins, I don't currently know of cases where I'd also advocate for singletons. Yes in most other cases singletons are bad but I'm sure there are some other cases where they do make sense. And you can't prove a negative.