Problem is that if Wasabi backend is not working there is no way to run Wasabi wallet even with your own node,

You're wrong, I'm running my own Wasabi backend on my own node. Anyone else can connect to it to synchronize too: https://coinjoin.kruw.io/

This means that if wasabiwallet.io is ever taken down, everybody's client software will need to update, with another backend.

You're wrong, there's no client update necessary, you simply just edit the 'Backend URI' text field in your client's global settings.