Alternate gamemodes/worlds such as deadman mode, a PvP world, an "oldschool" world or basically any other kind of gamemode that isn't the normal game would mean that we have to divide our playerbase. Things like minigames and the wilderness are already underpopulated, this would become even more of a problem if you divide the playerbase.
On the other hand, it's not guarranteed to get us more players at all. It's a trade off that's obviously not worth it. 200+ players are not just gonna come out of nowhere when an update is released. The only way we'll get players is by keeping them playing, it's a really slow process, but it's the only way.
Dividing our playerbase more is only gonna hurt the game overall.