It costs $xxx million in value being redistributed from the current holders of ether, permanent harm to the reputation of Ethereum and a strong leap towards a central governance model where a special minority get to choose which transactions are valid and which are not.
For those that want a quasi-centralised system where transactions can be undone, they should opt-in to a solution on the Ethereum application layer; these decisions shouldn’t be made at the protocol level. It’s clear from the volume of debate in this EIP-867 and EIP-999, the split sentiment on reddit and the results of the EIP-999 coinvote that many people are not interested in a system like this.
It is a contentious fork.