The same could be said for any bit of courtroom procedure. Why forbid a prosecutor from making an illegal inference, or entering illegal evidence, when you can

The same could be said for any bit of courtroom procedure. Why forbid a prosecutor from making an illegal inference, or entering illegal evidence, when you can just instruct the jury after-the-fact to disregard it? The jury doesn't just live in a vaccum. There are rules for what kind of evidence is admissible to them, and we don't just leave it to the jury to yolo it. Sometimes, those rules don't make 100% sense. That's the case with any set of rules (Especially ones surrounding legal procedure!). We set rules, because we think that they are a good idea most of the time. And most of the time, for most crimes, the tradeoff between trying a decades-old case, and of preventing the harassment of an innocent person over a decades-old-case leans towards the latter.