Serious question: Can't a government block it at the ISP level? They could also make out of country VPN illegal. How would BTC work in practicality under those conditions?
Semi-serious jokes aside (the block stream satellite can receive blocks but not broadcast), the protocol can operate under Tor in which the bitcoin core daemon is very well integrated. If worst come to happen, sneaker net is somewhat popular (https://opendime.com) and people have transacted using radio waves as a proof of concept (https://satoshi.radio.br/wp/).
Surely governments would target exchanges instead?
Without the dollars coming from retail investors, the value proposition for speculators disappears.
You could still trade bitcoin (slowly) at that point, but converting to fiat would be difficult.
Does that mean I can then exchange it directly with others without incurring a capital gains tax event?
Point is, shutting down legal exchanges won't happen in the US. Since you can't kill the underlying system or stop people from using it, you only make it impossible to tax.
You could broadcast it on any communications medium. You could theoretically run it over a global ham radio mesh network.
Lots of the network is already running on Tor, so you'd also have to shut that down.
You could embed the information in the pixels of cat pictures and broadcast it via instagram posts.
Unless there exists a global, horrifically authoritarian constriction on the information we are allowed to share, it's not going to happen.