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No matter what it looks like, the heart of any game is the game engine. I believe both DF1 and Elite game engines came from Unity. The DF1 version had a lot of bugs in it, and Unity has moved on to bigger and better things. Zup can't fix DF1 if he can't get the parts; he's holding it together with spit and bailing wire.
When the options are a.) throw all your time and money into a loveable, but ultimately unsalvageable game, or b.) save the franchise with a new, more easily upgraded and maintained game . . . which do you do?
Typically, I've seen two models for online game progression. You take a game like City of Heroes. The original game initially released on a subscription basis. Minor cosmetic additions and bug fixes came for free or were buyable options in the in-game store. Major new content bundles such as the City of Villains variant cost extra. The game had a good run (2004-2112), eventually allowing for free-to-play, before dying in 2012.
The second model would be the one followed by such games like Doom, or Call of Duty. You produce Call of Duty 1, you run it for all it's worth, and you develop CoD2 with that money. You release CoD2 it makes more money from not only your dedicated fans, but new players who want to see what all the noise is about. You leave the CoD1 servers open as long as they still support themselves, but you spend less and less of your resources on them, as you move on to CoD3,4,5, etc. Eventually, no one is playing CoD1 multiplayer anymore, and you shut it down.
I believe both those franchises had the benefit of significant development capital, and in CoD's case, they have drawn in massive revenue over the years. One advantage to the second model is that it is much easier (and cheaper) to integrate the newest game engine into the newest version of CoD because it is a brand new game within the CoD franchise.
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