But it is absolutely not cynical to assume that for each criminal person (forget politicians) who gets caught and charged, there are almost certainly more who didn't get caught or didn't get charged.
It is not cynical to assume that law enforcement has a less than perfect ability to prosecute criminals; it's just realistic.
Rather, it is naive to assume otherwise: What process, in any domain, has a 100% success rate?
Note: I'm not taking the "all politicians are corrupt" position, just "it is almost certainly true that corrupt politicians are often able to get away with their corruption". So, maybe we're not actually talking about the same thing.
Corrupt politicians, it seems, are like cockroaches: if you've confirmed the existence of one, then there are certainly several whose presence is as yet unconfirmed.