If you can't convert your bitcoin into fiat, then your only option is to use your btcs to purchase goods and services directly, which would further disincentivize businesses from accepting bitcoin as a form of payment. And if you can't send fiat to an exchange, you can't increase your holdings.

You can always buy and sell them in person for cash. Like how you buy drugs.

The point is the government can severely hurt Bitcoin adoption by making it illegal, but it cannot completely shut it down.

You could simply convert bitcoin into fiat in the 200+ other countries that are not the United States.

This would require you to open a foreign bank account, which you must tell the U.S. government about. Not telling them is a serious crime. See the Foreign Account Tax Compliance Act

If that doesn't kill it, they could further make any bitcoin transactions illegal.