Is it me or the density of accounts with Warzone membership has risen sharply? I don't remember people having a membership that much but this year, almost everybody I play with has one.

its all a fraud, dont believe what you see online. Membership is basically a scam IMO

The recent sale probably has something to do with it?

Maybe. Though I started noticing the increase even before the sale.

Inflation = People invest as long as the money has any value. 😅

Presumably (though I have no statistics to back this up) people with memberships leave the game less than the average player without membership.

If, say, the average non-member player has a 50% chance of leaving the game and never coming back every year, but the average member has only a 25% chance of leaving the game and never coming back every year, then over time the player base will come to consist of more and more members compared to non-members.