In the future, I think it's gonna get much more difficult for banks to trace where exactly the BTC came from.
It sounds like you're thinking as a bank, well they're not in difficult position, they're not at risk.
They have a rule like if the customer failed to show the evidence, the banks will give penalty which could make the tax double or triple than the actual tax. The banks can also have an option to seized the coins, so they don't need to work harder to handle this case, the customer is.
I'm sorry if I am making a bad analogy.
But why is this any different than paying for groceries in cash, and your cash payment being refused unless you can prove that your $20 bill did not come from illegal activities? That is not sustainable for banks to keep verifying.
I am the person who first bought the bitcoin from an exchange, so I can choose to show them where I got it from.
But I can pay my jeweler in bitcoin to receive a bar of gold for example.
If the jeweler has to declare the bitcoin on his balance sheet, and banks ask him where it came from, he can show them that it came from a customer.
That customer could have done something illegal like not paying taxes, or getting it from selling drugs.
But that does not mean that the jeweler did something illegal. He received bitcoin as payment and converts it to cash.
A $20 bill in my pocket could have passed through the hands of criminals, but that does not make it any less valuable or useful.