This topic has been discussed, requested, demanded and so on several times. It was on the survey they put up several months back and they did actually look into the possibility to implement an offline mode.
In the end it proved too difficult, or laborious to implement an offline mode due to the game being developed around the online requirements. So that idea was scrapped by the developers. What they did though was to reduce the services required to be constantly connected to, to in turn reduce the disconnects. But still it doesn't prevent you from loosing your progress in missions, if you loose the connection to all services.
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