Consider this: posts are historically locked because they are viewed by the community as being immensely useful even though they are glaring examples of what not to do on a Stack Exchange site.
If the answers on a historically locked question are veering towards being (or actually are) not useful anymore, then the justification for the post being historically locked should be revisited. If the value is not there anymore on a historically locked post, then by definition, it shouldn't be historically locked.
Naturally, if it's no longer historically locked, and still a glaring example of a post that is not a good fit for a Stack Exchange site, then it should be deleted.
If this is indeed the case, then you should raise the issue on Meta: it's generally better to get community consensus on these (as they would have been the ones to ask for a historical lock in the first place).