What kind of integrity are you talking about?

Merkle trees can prevent tampering after the fact, yes.

But if you include collusion, there's no way for the blockchain itself to know who is colluding and where they are so.

Smart contracts may be vulnerable or malicious.

Wallets can be emptied.

Centralized exchanges and similar entities still exist.

Policing systems are still needed, because as long as there is something of value and there is still "evil" in the world, someone will try to steal it or damage it.