Plotinus's thread were only numbered because they went beyond one because of popularity. There was no intent at the start to have a series that were focused on mostly trivial conversation. If you are going to argue this through examples, you'd do better to use Mafia threads as an example of spam threads that get pc. They can do 1000 posts a day.
Sure, but mafia threads aren't serial, they're merely long threads all about a single self-contained game. They don't even always reach 1000 posts if they don't go long (in fact, they usually were much shorter until we had a big influx of new players a few months ago who tended towards a much more talkative style of playing the game)
Like I said, the point of my post isn't really to argue over whether some example of a thread is too spammy to warrant PC or not, it's pointing out that it's really unclear what exactly the standard is, because it looks like it's something about serial threads but it's not all serial threads, not even within Off-Topic. I'm not trying to make arguments over whether something is spammy or not, because I don't even know if that's the actual standard used.