no one is responsible, that's the whole point. The thing runs by itself and game theory keeps it afloat - not a group of people. Else it's just like a company.
That said, the protocol isn't finished and you have people involved in maintaining and upgrading the protocol. Which is in no way forced down upon everyone: once they have an update everyone is free to choose to run it or not.
These people have influence (you could argue too much) about the future of the protocol.
> if your arbitrage trade takes away too much money from an account belong the core devs
This is not why these decisions were made at all, because some core dev instested in the DAO.
The decision-making and communities of Ethereum (this goes both for the clients, the blockchain, the foundation and the larger community) looks completely different today compared to 2015. There has been a lot of lessons learnt, debate and churn since.
If they same thing happened today, it'd play out completely differently.
See the Parity Multisig hack, for one.
Finally, it's a stretch to call the dao hack "arbitrage trade".
