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".