I think it's a little tougher than it sounds. You start arresting almost everybody after they leave office and you end up with incumbents that never want to leave office. Dictators for life. That's how the Roman Republic fell and that's what a lot of third world countries can look like. No thanks.
But if you let people slide on this kind of stuff, there's no incentive not to be corrupt. Then again, if you are going to prosecute people for everything, it can turn political really quick like Hillary's emails.
I honestly don't know where to draw the line. We prosecute people at the state and especially the local level all the time and the government doesn't end. Maybe we should start extending that level of law enforcement to the federal government, especially after some of the nonsense Trump and cronies pull off.
It seems like things that are outright corruption like taking direct bribes are prosecuted. Things like financially benifitting from insider knowledge, misappropriating government resources, especially travel, and influence pedaling are mostly ignored. And maybe that's right. I don't think we want to be prosecuting political opponents for every little thing because those political opponents will hopefully, eventually get back into power and they will do the same thing.