How can we use our bitcoin without fiat then?
I'm certain that when bitcoin got banned in a lot of countries, we would be using exchangers from a country where bitcoin isn't banned. There are also people buying bitcoins using their Paypal. You can make profit while converting your bitcoins then. Get it to your Paypal then cash the money out right away so he can't dispute it.
If we start to live a world where bitcoin is banned everywhere, people would just have to go back to person-to-person or peer-to-peer trading. We would do meetups and have to stay as anonymous as we can.
I really doubt that the government can control all of these but those are our only options when it happens.