IMO, you are overthinking it a bit. It's not that hard for teams to run multiple iterations of their budget/build, one with Cast opting in and another with an opt out.
Every trade has an element of risk. The risk being that the players you are trading for will help you achieve your goals better than the players you traded away. Let's say you are the GM of a win now team and Cast pushes the team over the top for a World Series title as long as he plays to the back of his baseball card. Is there an appreciable difference in your job security if you lose because you sat on your hands or pushed your chips in and the player didn't produce and you are stuck with him?
Of course the cost will be "baked in" to the risk but I think it will be a "beauty is in the eye of the beholder" situation. Higher budget teams wouldn't blink either way but you may bring in some lower or mid budget teams who are willing to roll the dice on the opt out.
While I wouldn't say it's a giant positive for the Reds, I'd say it's more positive than negative