Most of us trust a 3rd party to handle our stored financial assets. Banks, stockbrokers, etc. I don't think mainstream btc users will ever catch on to using offline wallets, and if they do it will be a less convoluted solution like Electrum.
From what I've read here and on reddit, offline wallets are as vulnerable to allowing btc to disappear as much or more than 3rd party online solutions with a good track record.