Giving government agents the ability to conduct warrantless dragnet surveillance on the population's private financial transactions is extremely dangerous, even if it is a tool government officials can use to combant malevolent criminal behavior.
This type of mass-surveillance concentrates power in the hands of the few, and gravely endangers both private property rights and political freedoms.
No one should have this power in a free society. Not you, not me, not the most trustworthy individual in society. And with all due respect to the countless men and women who honorably carry out their duty in law enforcement agencies, we don't even have law enforcement agencies with unblemished reputations, to be giving awesome illiberal powers to.
In the Silk Road case, two agents, from two different agencies, conspired to steal BTC, and they only got caught because of the mistakes they made due to the novelty of the financial technology. It's any one's guess how much corruption goes undetected.
Giving mass-surveillance powers to a select subsection of society is just asking for massive corruption and abuse.
Mass-surveillance makes government in general too powerful even ignoring employee-level corruption by those who administer the surveillance programs.
The government should not have so much information on the populace that it could enforce any conceivable law. There should be widespread use of privacy-enhancing technologies, like end-to-end encrypted communication, cash and cryptocurrency, so that some laws are hard to enforce, and the government's ability to micromanage people's lives is inherently limited. Absent that, dystopia is only one bad election away. Non-political checks on government powers, like widespread use of cash and its electronic corollaries, are an important institution of a free society, as a failsafe in case of a failure of the political system.