Make sure you set the game to only update on Launch, otherwise it will still download the update. This way the Workshop Connection remains. You can avoid the update by disconnecting the internet after you've logged in to Steam. This problem affects ALL steam games with a Workshop connection. Everything works fine ONLINE-mods, maps and asset items can all be found in game.
Please be realistic, this is a multi-million dollar business, not a neighbourhood supermarket. Cannot enable mods and workshop items in Cities Skyline I cannot access workshop mod items for Cities: Skyline when I am offline on steam. That would cost them million of dollars for breaking the contract, invalidate every single copy of the game already sold (Including yours!) and force everyone to re-buy the game, and force the company to invest millions in setting up their own Workshop system, because using a copy of Steam's system is a patent and copyright infringement. They can't just break a contract and switch system.
(Unless they're EA we all know EA has no common sense.) No company with more than half a gram of common sense would ever consider doing so. That would mean dropping a market of 20 million users. Paradox is not going to 'drop cooperation with Steam and make their own workshop'.
