This would mess horribly with jurisprudence constante, the principle that the legal system must above all be predictable, so that people can execute contracts or choose behaviors knowing the legal implications. But since we are already, despite our claims to the otherwise, ok with using things like retroactive laws, how problematic this would be in practice is hard to ascertain.
I know about that principle, yeah - even EY mentioned it around here somewhere - but in practice the modern judiciary branch (to my eye) is very chaotic and even corrupt anyway (enforcement of copyright is, in practice, enforcement of insanity, mass murderers can more or less walk away if their fans are threatening enough, the policy towards juvenile offenders seems to be designed for destroying their lives and making them more dangerous, etc, etc), so indeed it's largely only predictable in its consistent badness. (That applies to more or less all modern ...
If it's worth saying, but not worth its own post, even in Discussion, it goes here.