Banning crypto (or drugs or gambling or alcohol or whatever) is one thing. Actually enforcing that ban is another.
And since there is no physical product or evidence it's actually much easier, my friend from Amsterdam will have a lot of trouble bringing me weed, he will have zero difficulty sending $100 worth of BTC to an address I give him.
The idea that you can just ban things and they will go away should have died with alcohol prohibitions. I guess at least no one will get killed in a war on crypto...
And even if it doesn't, your friendly local drug dealer, who may already be trading crypto to purchase product, will presumably offer exchange services. No one needs an exchange they can just charge a spread between people wanting to buy and those wanting to sell. That's how money change already works after all...
And you're right about Monero!
US has tried the ban hammer both for alcohol and then for drugs, how has that worked out for them?
That would just create a massive underground black market, this time even more pernicious because it's international, with state-level actors.