Basically this. Anyone who is surprised by this has been paying literally zero attention to how these things usually go. The majority of the time when a game explodes that much, this happens. Sometimes to a lesser extent, sometimes to a greater one. A good chunk of the people who buy the game in the first place buy it to play with their friends, and when their friends move on to another game, they will too.

While you will have games that are the exception, such as PUBG which has had massive a player count for forever. they are indeed the exception.