The way you actually prosecute theft and fraud is by looking at the ledger of transactions to see what happened. Money laundering is any deliberate attempt to make tracing those transactions more difficult. Because it hides the evidence of theft or fraud, we criminalize it the same way we'd criminalize theft or fraud, because it's far easier to prove the cover-up than the actual crime. Furthermore, once you've shown evidence of a cover-up, that creates suspicion to start dissolving privacy to prove the actual crime. Dragnet surveillance is illegal[0], but surveillance with reasonable suspicion is not.

In the counterfactual world where money laundering is explicitly legal, crime becomes far harder to prove, the government is far less useful at stopping crime, and people spend more of their time inventing parallel structures to stop crime that are far less accountable than the government is, until we've sleep-walked into anarchocapitalism.

None of this excuses 'debanking' of course. In the case of debanking, you aren't being accused of the crime of money laundering, the bank just thinks you might be, so they summarily execute your account in the same way that Google executes[1] spammers. The thing you need to be angry about is the weaponization of freedom to associate, and you need to be calling for common carrier regulation rather than legalizing financial crime.

[0] Theoretically, at least - the CIA and NSA have fought tooth and nail to get full take, which is disgusting

[1] Metaphorical, at least until 2035 when Google starts livestreaming executions of known spammers