Who said we need decentralization? Adding that requirement drastically increases costs without providing any practical gain. Card networks provide near instant payment processing with very low fees. Need irreversible payments? Simply use the same architecture without implementing chargebacks.
Take your money before anybody asks for their money back. As a merchant, you will have signed a contract with the payment processor agreeing to certain terms, and they cannot change those terms unilaterally. That scenario is as useless as imagining the main mining groups deciding to roll back to an earlier block.
They can if they control a majority of mining resources. They simply start mining new blocks from an earlier block. This happens all the time when there is a race. The loser has to discard their block and start mining off the winner.
The only thing that is stopping this is the miners' incentives, which is the exact same thing that stops contract violations.
Mining isn't exactly that centralized and even if it were they'd only really be able to roll back the most recent block. That's nothing like general chargebacks, as it would be one-off thing and only be in effect for 10 minutes, and is heavily disincentivized as it would destroy the reputation of the miner.
The difference with legal contract enforced systems is they aren't. No one is going to build a system that restricts it's owner legally, and furthermore legal rules are leaky abstractions that have workarounds and spotty enforcement.
