which is why people have warned several times already that time-gating essential items around events will hurt populations; I noticed Destiny doesn't use that and has higher population, and Ephinea which dwarfs everything else (222 right now!) ditched that some time ago.
But that raises several other questions over why Ephinea is massively more popular than everyone else.
Also, PSO2 didn't kill Ephinea's large population, so we can't rely on that.
I would note that popularity is fundamentally about what most players want, and the signs are pointing to several factors including when core items are available.