The argument against this is simple. All of the seats in a given class of service on a given flight are roughly the same in that they all transport one between two points. The greatest fare differentiator is flexibility. A fully flexible ticket costs a lot more than one which is wholly inflexible.

But, why on earth would I pay the premium for flexibility if I could have it for free? As a consumer, I purchase the cheapest ticket which fulfills my needs. In this case, transport between MAD and LHR with flexibility. If that can be had at OP's inflexible price, that is what I am willing to pay and I would be mad to pay the premium for flexibility.

The US carriers fell down this rabbit hole with Same Day Change and Same Day Standby, either for a small fee or no fee (in the case of those with status or high status). It has wreaked havoc for those with flexible tickets and led to all manner of people gaming the system.

Just as people ask why BA will send out a CW or F cabin with empty seats rather than UG someone as a courtesy, the answer is that protecting the premium brand, whether it is a soft product such as a cabin or a ticket type such as flexibility, is all the same.