I think air gap does not guarantee that funds are never stolen.
The reason is that you don't know what is inside generated transaction bytes that you copy&paste to an online computer to broadcast. Potentially if myetherwallet site is hacked and you downloaded hacked version, it can attach your private key and password to transaction data. Since your are encoding it and do not know what is inside, you will gladly copy&paste your transaction to online computer and send it, thus giving control to your wallet to everybody.
I wonder if there is any solution to that risk, like inspecting transaction bytes to make sure the size is not greater than certain number, so that no extra data can be fit. Or may be creating restricted wallet contract that can only send ether without any attached data.
And then good broadcasting software like myetherwallet should inspect length of tx bytes and warn user if it's too long for just sending funds.