There were unlockables such as music and Sin was unlocked when u had 200K ingame currency, who can also be bought.
Basically make it so u can gain it offline then sync with servers.
You're still missing the point.
If you play offline, you can use trainers.
If you use trainers, you can easily beat Hard/Hell with everybody.
16 x 30k (10k per hard survival, 20k per hell survival) = 480k
This doesn't include level up bonuses to FM, just the straight up reward money for completing survival with each character.
If each DLC character costs $6 (projected based on previous zenny knowledge) or 100k fight money, by the time you factor it in, people who play offline and still get fight money have just gotten over $30 in free content.
My Xrd knowledge is not very deep, but I would guess that Sin is just an unlockable boss character, not a DLC character. So Xrd using in game currency to unlock extras already in the game is nothing worth policing/requiring an online connection for. If Sin was a $7 Aksys DLC character, I don't think you'd be able to earn your FM offline for it.