I think it could be argued that case law is a prevalent form of ex post facto law in common law systems, and so constitutes an additional justification for stat

I think it could be argued that case law is a prevalent form of ex post facto law in common law systems, and so constitutes an additional justification for statutes of limitations. Many acts are ambiguously criminal, until clarified by a test case. Legal decisions are also often made in the context of time, taking into account what is commonly considered offensive, what a person is presumed to know (in establishing mens rea). It's difficult to try some crimes with a modern perspective and take into account what was considered acceptable in the past.