The auction house was "never the driving factor" in making D3 always-online.
By Tim Colwill on September 19, 2013 at 12:45 pm
Blizzard’s announcement that they would be shutting down the auction houses for Diablo 3 had many excited that this was just the first step towards an announcement of a long, long-awaited Offline Mode. Unfortunately, it looks like this was not the case.
“We do not have plans to implement an offline mode,” said Blizzard to Rock, Paper, Shotgun. “While the always-online requirement made the auction house possible, the auction house was never the driving factor in our decision to make the PC version of Diablo III require an Internet connection.”
“The game was built from the ground up to take full advantage of Battle.net, which provides a number of important benefits, including persistent server-side character saves, a seamless PC multiplayer experience, cheat prevention, and Real ID and BattleTag social features.”
Source: to Rock, Paper, Shotgun