People that want to play single player offline most likely don´t like to play with friends or strangers. They like to play solo. And so even if they hack and get the best possible loot in the game they wont be able to sell it online or farm for gear.
It doesn´t make sense to me that people would like to play offline to then go play online. If you want to play online is because you have or will have friends to play with and use the auction houses. If so then stick with online even if there is somewhat horrible lag atm. Afterall there isn´t a single game online that does not suffer from any kind of lag unless the server is on the building next to yours.