Maybe I'm missing something, do you often communicate via email to your bank?

I know I do not. Pretty much never ever. That being said if I were to have to, rather than via their website or phone call, I can update that data on my bank account. I'm sure I must go through few (security) hoops to do it but I doubt it takes me more than 5min. In such case... I would put whatever email and I need to switch because I don't want to pay for that domain anymore then I would just update my contact information there.

TL;DR: I also use my own domain name for banking but in my case I'd argue it doesn't matter much.