As I've pointed out a few times previously- the only real reason an "offline" mode wasn't possible was because of the BGS core (single shared galaxy) and that seed changes wouldn't have been reflected properly (syncing) if people were using offline clients. (which is ultimately why everyone plays "online-connected", even when in Solo) Had the "single shared galaxy" and BGS not been central to the function of this game, it would have been entirely possible to release it as an offline game. Personally I would not at all have minded if they had given each customer this choice.
Arguably "DRM" could be linked to this, too- although I think the former was the most definitive reason for having an online-only component. The latter was just icing on the cake.
Having said that, I truly believe it's possible for Frontier to change each client to seed it's own "BGS" by replacing the core code once they've decided to shut the servers down. Wouldn't be all that difficult to redirect BGS change requests and basically the client machine becomes the BGS server.