>Either people who get away with it undetected in the very short term are so good that they manage to suppress all evidence completely and it never comes out (which seems an implausible binodality in outcomes)

Why do you think that's implausible? What evidence of election fraud would you expect to be left behind that would be difficult to dispose of?

>is allowed to stand by corrupt institutions despite being widely decried.

By whom?

>If it were, you'd expect there to be examples of cases undetectable in the short-run but later discovered.

Why would that be expected? Many crimes are much harder to prove if the perpetrator is not caught in the act. Why would you not expect election fraud to be among them?