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and its not because its a themepark and sandboxers hate themeparks. And its not because sandbox games are all indie games.
For themeparks. Its because people complain that there is no endgame content within 1 month (or some complain 3 months) after release. Stating at the same time, that WoW has TONS of endgame content and way more stuff to do. While forgetting that WoW endgame content sucked until Burning Crusade and Wrath of the Lich King (I liked fighting Arthas, for lore reasons...)...future expansions added lots of content (but I stopped playing after cataclysm)
That means NO themepark can ever succeed. As I've seen MMOs die SO fast due to not nearly as much endgame content as WoW. People don't even give the game a year (like they did with WoW), before mass leaving. No themepark, no matter how much money is spent, can ever catch up to WoW at release. Now, since WoW only comes out with 2 patches a year and an expansion every 1-2 years...MMOs can come out with much faster updates (every month like RIFT), and end up with more content. But most people don't give MMOs the chance to do so, and just end up going to WoW where there is tons of content.
Then there is EVE. The opposite genre of WoW. No sandbox game can ever compete with EVE. EVE nails PvP, so why play a clone of EVE's pvp system? EVE nails diplomacy, which has taken YEARS to develop. No sandbox game can ever spend enough time before its release, to develop diplomacy. On top of that, like WoW...EVE has TONS of content. So much to do. Now PvErs may not like EVE...but Ryzom is a PvE sandbox, and its dead...many polls on MMORPG and elsewhere, most people won't play a pure PvE sandbox. They want at least SOME PvP. However, even EVE PvE got way better once they introduced events, wormholes and all that jazz...
No sandbox MMO can ever catch up to EVE in terms of stability (including lack of bugs) AND content. I've seen many sandbox MMOs come out and people complain there is no content (or lack of it), or EVE is better at x and y or any number of things.
So, in other words...unless you come out at RELEASE with as much content as EVE and WoW (which is impossible), most people will leave before the year is up and not give it a chance. How do I know this? I've seen this almost everywhere, on official MMO forums, MMORPG.com and other online game forums...most people leave because of lack of content at release.
People keep hoping for THE ONE MMO..but they don't give THE ONE a chance. Instead they leave within 3 months due to lack of content and go back to WoW and EVE. Doesn't matter if its a sandbox or a themepark, you need as much (and high) quality content as the two big MMOs in their genre.
(edit: Fixed a spacing error, added a line at end of the post)
My Skyrim website (mods, screenshots, stories and other games I play(ed)