In a purely single user game, there would really be no value in having a 400 billion star system universe - even playing continuously 24x7, never visiting any system twice, you'd need multiple lifetimes to visit every system in the galaxy, never mind actually taking the time to get any enjoyable gameplay in while doing it.
That also applies to the current game. In the best case stars outnumber players one hundred thousand to one.
FD have also talked about giving away the secrets of the galaxy
Which doesn't make any sense given that the galaxy is procedurally generated, and simply changing the seed would give us an infinite number of variations.
gameplay not matching the richness of the online experience etc, and the effort required to develop an offline universe that would match expectations
Which seems to indicate a seriously flawed design.
If even that is too much effort, how about documenting & publishing the solo-online subset of the client-server API and incorporating an option that permits alternative connection to a 'localhost' server for solo play into the client? That server API will probably be documented at some point by a geek with a network sniffer & the patience to reverse engineer it, so get your retaliation in first... how would this help? Well, if the community can learn what the client sends to and expects from the server in solo online play, the community can develop its own local VM based server... can you imagine what the OOlite modding community might build given the opportunity to build an ED client compatible local VM based server for ED?
This would be awesome (and if the game ends up having a large enough community the servers are going to be reverse engineered any way), third party servers would be great for the longevity of the game (independently of Frontier's), but they'd probably dismiss it claiming fears of "hacking" or other excuses (Braben already expressed worries about the game "branching" in an answer about modding).