Your email account is the golden key to all other accounts that send "forgot password" links to it.
But what happened was embarrassment, not their life savings being wiped out.
a) There is a nonzero probability that your bank can be socially engineered using information obtained from compromising your email account and anything that trusts it. b) An email account compromise implicitly means every service that resets/recovers through it has to be rekeyed. The subsequent cleaning of the stables can be messy, lengthy, and itself somewhat risky.
In particular, if you haven't already, enable MFA. If your email provider does not support MFA, change your provider.
This is 2016, we have password managers. Using different passwords for each site shouldn't be any more difficult than if you had not. Even using the built in ones in your browser of choice is better than not using one at all and makes using site specific passwords easy. Chrome even has a built in password generator for you, I assume this is using your operating systems CSPRNG (or BoringSSL's?) although I'm not 100% sure about that.