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We all know that themeparks post-WoW have a problem with the retention rate of players (the already discussed "themepark trap"), that they start with a big burst in subscriptions and after some months (when the people reach the "endgame") the playerbase falls fast.
Well, i think sandboxes without ow meaningful pvp can have a even worse fall in the playerbase than these themeparks, since:
1- Themeparks can put their "carrots" in instanced areas, which allow them multiply by hundreds or thousands the real in-game "content" available to the players. Sandboxes in the other hand will have to provide all their pve content (at least the best) in the open world, which restricts dramatically the amount of "pve content" (that works as "tools" to pursue the "carrots") available to players.
2- Outside the structured and "dramatized" challenges provided in instanced dungeons or raids, there are not many alternatives left to provide the players with funny and cool combat challenges without pvp. The openess of epic bosses to the world without the possibility of players dispute it through fights between them would preclude the tactical and challenging aspect of fighting bosses. These fights would be a zerg of thousand of players trying to tag or take the most credit from a easy boss kill. With open pvp the players can at least have the challenge of expelling others players, keeping them away while a minor group kill the boss.
3- Without instanced content, the gathering of itens, farming of mobs and killing of elites/bosses would occur through a zerg of players in a crowded open world area. The whole thing would be to decide who "tags" the itens first or who can hit more the targets, earning more credits. Even if the developers make the respawn rate very high (or instant respawns), the whole proccess would be far dumber and boring than themepark instances.
4- Without ow pvp, many metagame content arised from players disputes that originate political, diplomatic, social, moral and economical complexities (e.g: carismatic power or leadership, spying, sabotage, betrayals, big rivalries and fellowships, reputations with real consequences, etc) would be damaged or even wouldn't exists. The mmo would be only a bunch of players gathering itens and building stuff that would have little or none use besides the cosmetic, without challenges and still jammed by the crowd in the proccess. Anyway, this would be way less fun than themepark instanced stuff and the created things (buildings, gear, etc) would have little use and meaning.