Despite this being true, it is important that people are prosecuted for the "all" the wrongs they did and not just the "easy" ones. It isn't much of a justice system if you can get away with "hard to prosecute" crimes.
As a counterpoint, IMO it's important that people are only prosecuted when there is sufficient evidence for a likely and safe conviction (otherwise at best we will drag everyone through expensive court battles which don't lead anywhere, and at worst we will end up with unsafe convictions).