Are you saying that it is impossible for bugs or vulnerabilities (open to exploitation by attackers) in the cryptocurrency's software code to exist? And that there are no points of failure or ways the decentralization can be eroded to create such vulnerabilities.
The Blockchain is open ledger. People can see the code and can see the vulnerabilities. Its not that its impossible to build that sort of stuff into code. Its that its impossible to hide it unless you refuse to publish the code in which case nobody is going to trust you or your code.