How do people deal with the risk of a Mt. Gox-type event, and what assurances are there that it won't happen again?
The security risk at exchanges arises when people start placing coins into wallets the exchange controls, rather than the users themselves. It's not a bitcoin vulnerability, and can be easily avoided, but many people will choose not to avoid it, so exchange hacks will keep happening.
You can read more about the Mt. Gox disaster here: https://www.wired.com/2014/03/bitcoin-exchange/