Back in the early 00's, they used to refer to games being potential "Halo killers". As in games that could break the stranglehold Halo had on the genre.
COD succeeded mainly because of its setting. It was... well, modern warfare. Took place in modern time with modern weapons in a fairly realistic setting. It allowed its players to put themselves into the action and live out their fantasies of being a soldier. It succeeded specifically because it *didn't* go for a sci-fi setting.
So, when it goes into space, it's not giving its fans what they want. It's basically the same as if you gave Link an AK-47, if you had Skyrim take place in modern day Hawaii, or you had Star Fox flying missions in WW1. Or if you gave Dante Black hair, made him half angel, and made him fight an evil corporation. Or if you had Banjo and Kazooie riding around in a lego car. Or if you have the Prince of Persia cussing up a storm.
Point is, there is nothing wrong with making a game where you shoot aliens. But, if you completely disregard what your fans want from the series, don't expect them to be happy.







