The problem with that theory arises when someone "forgets" to attach a fee to the tx. Try sending a no fee tx these days. With the current tx volume on the network, it is damn near impossible to send a tx without fee. It will get returned to sender within a day or so.
transactions without fee fastest I've ever done takes about 6 hours. but now when I try it again,
it almost becomes impossible.
If you both are so sure that a transaction without a fee will be returned to you, then I'd like to propose a challenge to you. Let's see you put your money where your words are:
I'll provide you a bitcoin address of mine. You send 1 BTC to that address without a fee. Then double spend that same input back to an address that you control (or wait for it to be "returned to sender").
If you can keep the transaction from being confirmed to my address, I'll send you an extra bitcoin as a reward for your effort.
If you fail to keep the transaction from being confirmed to my address, then I get to keep the bitcoin.
So, if I'm right, I gain 1 BTC from you. If you're right, you get your bitcoin back and you gain 1 BTC from me.
Should be the easiest $250 you'll ever make. Right?