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I started this at the pub but I think this question may be better here.
Why wouldn't developers make other standalone games using the code from their MMO? They already have an engine, combat system, characters, armor/clothing, and a world to build upon. Keeping everyone in the same game is very important, the next best thing is to keep them in the same universe.
There is so much in MMOs, it almost seems like a waste for only one game when a small investment could fund another game to appeal to other markets. Genres like MOBA, ARPG, lobby battlegrounds, single-player adventure (with group support) - a creative team could go crazy thinking of possibilities.
These games could be priced moderately or f2p with cash shop that wouldn't even affect the original MMO. SWTOR could make their own version of Battlefront, GW2 could make a cool siege game (similar to War of the Roses or Chivalry), or a MMO could just make a single-player survival/builder game like Minecraft. Even if these games are horrible (which would apply more to the developers rather than the idea of making a game from a game) they could still make a profit.
Warhammer already tried something like this with Wraith of Heroes. That game was awesome fun until they made a few bad design decisions and started a crappy p2w shop (again, fault of company). Blizzard may not be using the same code but they do reuse vital designs such as; characters, abilities, and art to make other games like Hearthstone and Heroes of the Storm.
The community alone could make some pretty cool games if companies allowed us to mod their games - entire genres have been created like this - Does anyone else see the potential here!?