It's not that simple. Once a transaction gets confirmed on one chain, it's permanently able to be spent on the other. If, due to fees or other reasons, that transaction is not confirmed quickly, the merchant may not be able to return the funds.

Also, automated system may have no way of handling it if those automated systems are not upgraded. Which means a merchant could end up with thousands of payments that they have to manually fix. And, those merchants will be forced into adopting the new software to correct the issue, they will have no choice to o ignore it.

It FORCES every deployed system in the entire ecosystem to upgrade, even systems that don't consent to the change. That's absolutely unacceptable.