Could Gov't block conversion to FIAT in future?

Newcomer to bitcoin here. I'm convinced about the utility and value of Bitcoin and the importance of HODLing but like any asset you need the ability to get in AND OUT of any asset for it to be adopted and be truly useful. For instance, I would never buy a stock or asset if I knew I was locked in for life or forbidden from selling no matter how lucrative it may be. My question is this, Can't the U.S. Government just block any exchange or customer trying to convert BTC to USD for example? What's to stop them from shutting down a RIVER or Strike or imposing a ridiculously low sell limit like 200 bucks a day or a week or something like that? I'm already hearing stories of people trying to cash out more than 10k of BTC and getting Flagged in the process (although to be fair this occurs with FIAT transactions as well). How would selling large amounts of BTC to fiat work? (tens of thousands or even hundreds of thousands of BTC in USD for example) I'm concerned about this. If anyone can assuage my fears that would be helpful and appreciated. Pardon my naïveté in advance.

P.S. Please not PM me as I do not own anything.