The idea of the cancelled train policy is to remedy the impact of the cancelled train. Are you really suggesting that it might be appropriate to sell an off-peak ticket for a cancelled train on the basis that cancelled train policy means customers can now travel on the 'peak train'? The policy is for journeys already booked prior to the cancellation coming into effect. It does not create a lifting of restrictions or price easement for trains either side.
A hotel has two rooms available, one very nice room at £120 and one average room at £80. On day zero, a customer wants to book a room. On day one, there is suddenly a leak in the average room such that it can no longer be made available. If the customer books on day zero, they get that room for £80, then on day one they get moved into the nice room at no further charge. If the customer books on day one, that room is no longer available and they would need to pay £120 to get the last remaining room, the nice room, they would not be able to get that room for £80.
Note that the policy is written on the basis of customers 'using their existing ticket' on alternative services. Regardless of TOC interpretation of the policy in terms of TCS vs Darwin vs on-the-day, it will always provide a means for customers to use their existing ticket in more flexible ways, not to provide workarounds for on the day ticket sales. Policing this is a different conversation, but I suspect if any TOC identified this to be happening, they'd crack down on it pretty swiftly.