As with most things the concept of "decentralization" is shades of gray. While there is no central controlling entity in the Ethereum network, there are people behind it and using it and people tend to congregate into groups with power structures.
If you want to force a transaction to be reverted you need to either create a cataclysmic event prior to the event you want changed, or you need to own billions of dollars in hardware and spend tens if not hundreds of millions on electricity currently to overpower the network via 51% attack.
If you want to force a transaction to be reverted you need to either create a cataclysmic event prior to the event you want changed, or you need to own billions of dollars in hardware and spend tens if not hundreds of millions on electricity currently to overpower the network via 51% attack.